beacon-client: fix concurrency issues in tests

[iurii-ssv]

I've noticed there are couple of issues:

• t *testing.T is not meant to be used concurrently:

// 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.
//
// The other reporting methods, such as the variations of Log and Error,
// may be called simultaneously from multiple goroutines.
type T struct {

• and there is also this error (see https://stackoverflow.com/questions/57828645/how-to-handle-superfluous-response-writeheader-call-in-order-to-return-500 for more details):

2025/02/27 10:15:16 http: superfluous response.WriteHeader call from github.com/ssvlabs/ssv/beacon/goclient.mockServer.func1 (goclient_test.go:264)

this PR should address both

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.9%. Comparing base (13fbc84) to head (8985884).

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