[UnitTests] Fix order-dependent synchronous decorator tests

The decorator tests which used synchronous decorators started failing
if they were run alone (before any of the other tests).

This broke in
d469daa
, when I enabled `GTMStandardUserAgentProvider` by default in fetchers
which don't otherwise specify a user-agent provider.

The issue is that `GTMStandardUserAgentProvider` itself is
asynchronous, but only the first time it's ever invoked (after that,
it caches its result in a process-wide cache).

The synchronous decorator tests assumed they'd always stay synchronous,
but this wasn't the case if they used `GTMStandardUserAgentProvider`
and were the first tests to run in the process.

This fixes the issue by always providing a `GTMUserAgentStringProvider`
for the synchronous decorator tests.

Tested:
  `swift test` passes.
  Also ran test
  `-[GTMSessionFetcherServiceTest testMultipleDecoratorsSynchronous]`
  by itself and ensured it failed before this change and passed after
  this change.

UnitTests/GTMSessionFetcherServiceTest.m

@@ -1185,6 +1185,10 @@ - (void)testSingleDecoratorSynchronous {
       [GTMSessionFetcherService mockFetcherServiceWithFakedData:nil fakedError:nil];
   [service addDecorator:decorator];
   GTMSessionFetcher *fetcher = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  // Ensure this test always runs the decorators synchronously -- if it's the first
+  // test to run, then the `GTMStandardUserAgentProvider` will asynchronously add
+  // the User-Agent.
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
@@ -1244,6 +1248,7 @@ - (void)testSingleDecoratorSynchronousAddHeaderOnRetry {
   fetcher.retryEnabled = YES;
   fetcher.minRetryInterval = 0.01;
   fetcher.maxRetryInterval = 0.05;
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
@@ -1267,6 +1272,7 @@ - (void)testSingleDecoratorSynchronousWithError {
       [GTMSessionFetcherService mockFetcherServiceWithFakedData:nil fakedError:nil];
   [service addDecorator:decorator];
   GTMSessionFetcher *fetcher = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher beginFetchWithCompletionHandler:^(__unused NSData *fetchData, NSError *fetchError) {
     XCTAssertEqualObjects(fetchError, error);
@@ -1298,6 +1304,7 @@ - (void)testMultipleDecoratorsSynchronousWithErrorShouldNotCallSubsequentDecorat
   [service addDecorator:decoratorA];
   [service addDecorator:decoratorB];
   GTMSessionFetcher *fetcher = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher beginFetchWithCompletionHandler:^(__unused NSData *fetchData, NSError *fetchError) {
     XCTAssertEqualObjects(fetchError, error);
@@ -1323,6 +1330,7 @@ - (void)testMultipleDecoratorsSynchronous {
   [service addDecorator:decoratorB];
   [service addDecorator:decoratorC];
   GTMSessionFetcher *fetcher = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
@@ -1349,6 +1357,7 @@ - (void)testMultipleDecoratorsSynchronousFetchedDataAndError {
   [service addDecorator:decoratorA];
   [service addDecorator:decoratorB];
   GTMSessionFetcher *fetcher = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcher.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectation = [self expectationWithDescription:@"Fetch complete"];
   [fetcher
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
@@ -1376,20 +1385,24 @@ - (void)testSingleDecoratorSynchronousWithDifferentHeadersForEachRequest {
       [GTMSessionFetcherService mockFetcherServiceWithFakedData:nil fakedError:nil];
   [service addDecorator:decorator];
   GTMSessionFetcher *fetcherA = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcherA.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
+
   XCTestExpectation *fetchCompleteExpectationA =
       [self expectationWithDescription:@"Fetch complete"];
   [fetcherA
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
         [fetchCompleteExpectationA fulfill];
       }];
   GTMSessionFetcher *fetcherB = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcherB.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectationB =
       [self expectationWithDescription:@"Fetch complete"];
   [fetcherB
       beginFetchWithCompletionHandler:^(__unused NSData *fetchData, __unused NSError *fetchError) {
         [fetchCompleteExpectationB fulfill];
       }];
   GTMSessionFetcher *fetcherC = [service fetcherWithURLString:@"https://www.html5zombo.com"];
+  fetcherC.userAgentProvider = [[GTMUserAgentStringProvider alloc] initWithUserAgentString:@"Lynx"];
   XCTestExpectation *fetchCompleteExpectationC =
       [self expectationWithDescription:@"Fetch complete"];
   [fetcherC