Add MSVC coverage for Constant and UnitSymbol

Some comments on the vcpkg PR to upgrade Au to 0.4.0 made me suspicious
that we lacked coverage here.  I'm going to add it and see if it breaks.

.github/workflows/single-file-build-and-test.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: Build single-file package
         shell: cmd
-        run: python tools/bin/make-single-file --units meters seconds --version-id NA > au.hh
+        run: python tools/bin/make-single-file --units meters seconds --constants speed_of_light --version-id NA > au.hh
 
       - name: Build and run test
         shell: cmd

single-file-test.cc

@@ -26,6 +26,8 @@
 // dependencies outside of the C++14 standard library, and the single-file package of Au itself.
 
 using namespace au;
+using ::au::symbols::m;
+using ::au::symbols::s;
 
 // This ad hoc utility is a stand-in for GTEST, which we can't use here.
 template <typename ExpectedT, typename ActualT>
@@ -42,6 +44,7 @@ int main(int argc, char **argv) {
     const std::vector<bool> results{
         {
             expect_equal((meters / second)(5) * seconds(6), meters(30)),
+            expect_equal(SPEED_OF_LIGHT.as<int>(m / s), 299'792'458 * m / s),
         },
     };
     return std::all_of(std::begin(results), std::end(results), [](auto x) { return x; }) ? 0 : 1;