Fix flaky TestHandleRefreshWithoutProxyServerStart test

[makhov]

Fixes #910

expectedReply is created before we start iterating over test cases, and the clock is ticking, so the server can respond in the next second.

Before the change, it hit the issue 1-3 times in 1000 runs:

 $ go test ./internal/proxy -run TestHandleRefreshWithoutProxyServerStart -count 1000
--- FAIL: TestHandleRefreshWithoutProxyServerStart (0.00s)
    refresh_handler_test.go:198: 
                Error Trace:    /Users/amakhov/www/centrifugo/internal/proxy/refresh_handler_test.go:198
                Error:          Not equal: 
                                expected: centrifuge.RefreshReply{Expired:false, ExpireAt:1733732303, Info:[]uint8(nil)}
                                actual  : centrifuge.RefreshReply{Expired:false, ExpireAt:1733732304, Info:[]uint8(nil)}
                                
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -2,3 +2,3 @@
                                  Expired: (bool) false,
                                - ExpireAt: (int64) 1733732303,
                                + ExpireAt: (int64) 1733732304,
                                  Info: ([]uint8) <nil>
                Test:           TestHandleRefreshWithoutProxyServerStart
                Messages:       http
FAIL
FAIL    github.com/centrifugal/centrifugo/v5/internal/proxy     4.763s
FAIL

With the fix there is no failures in 10000 runs:

$ go test ./internal/proxy -run TestHandleRefreshWithoutProxyServerStart -count 10000
ok      github.com/centrifugal/centrifugo/v5/internal/proxy     68.674s