skip impersonation test when using emulator

Author: scotthart

google/cloud/pubsub/integration_tests/subscriber_integration_test.cc

@@ -58,6 +58,10 @@ using ::testing::ElementsAreArray;
 using ::testing::IsEmpty;
 using ::testing::NotNull;
 
+bool UsingEmulator() {
+  return google::cloud::internal::GetEnv("PUBSUB_EMULATOR_HOST").has_value();
+}
+
 class SubscriberIntegrationTest
     : public ::google::cloud::testing_util::IntegrationTest {
  protected:
@@ -352,6 +356,7 @@ TEST_F(SubscriberIntegrationTest, PublishPullAck) {
 }
 
 TEST_F(SubscriberIntegrationTest, PublishPullAckWithImpersonatedCredentials) {
+  if (UsingEmulator()) GTEST_SKIP();
   std::string iam_service_account =
       google::cloud::internal::GetEnv(
           "GOOGLE_CLOUD_CPP_PUBSUB_TEST_IMPERSONATED_SERVICE_ACCOUNT")