ARROW-7701: [FlightRPC][C++] disable flaky MacOS test

Unfortunately, gtest doesn't have a good way to do platform-specific test skips, so this disables the problematic test entirely for now.

Closes #6391 from lidavidm/arrow-7701 and squashes the following commits:

7802b03 <David Li> ARROW-7701:  disable flaky MacOS test

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/flight/flight_test.cc

@@ -1247,7 +1247,13 @@ TEST_F(TestBasicAuthHandler, CheckPeerIdentity) {
   ASSERT_EQ(result->body->ToString(), "user");
 }
 
+#ifdef __APPLE__
+// ARROW-7701: this test is flaky on MacOS and segfaults (due to gRPC
+// bug?)
+TEST_F(TestTls, DISABLED_DoAction) {
+#else
 TEST_F(TestTls, DoAction) {
+#endif
   FlightCallOptions options;
   options.timeout = TimeoutDuration{5.0};
   Action action;