*testing.T.Fatalf() is called in sendTestMessageAsync() from mutiple goroutines

https://github.com/AdguardTeam/dnsproxy/blob/2f7a50c830677fd6fed48a071a5f8f4ff563c1a8/proxy/proxy_test.go#L751

https://github.com/AdguardTeam/dnsproxy/blob/2f7a50c830677fd6fed48a071a5f8f4ff563c1a8/proxy/proxy_test.go#L734

The golang comment says that `Fatalf()` `must be called only from the goroutine running the Test function.`

https://github.com/golang/go/blob/926994fd7cf65b2703552686965fb05569699897/src/testing/testing.go#L659

```
// T is a type passed to Test functions to manage test state and support formatted test logs.
//
// A test ends when its Test function returns or calls any of the methods
// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods, as well as
// the Parallel method, must be called only from the goroutine running the
// Test function.
```