ms := startMockServer(t)
	defer ms.Stop()

	// Configures the mock server to return error after the first request.
	ms.ReturnErrorAfterFirstNumHits = 1
	ms.ReturnSuccessAfterFirstNumHits = 3

	mockCertURL := ms.URL + "/oauth2/v3/certs"
	_, err := r.GetPublicKey("", mockCertURL, testRequestTimeout)
	if err != nil {