Add and fix _metadata_flags test

src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi

@@ -260,6 +260,7 @@ cdef void _call(
         on_success(started_tags)
     else:
       raise ValueError('Cannot invoke RPC: %s' % channel_state.closed_reason)
+
 cdef void _process_integrated_call_tag(
     _ChannelState state, _BatchOperationTag tag) except *:
   cdef _CallState call_state = state.integrated_call_states.pop(tag)

src/python/grpcio_tests/tests/unit/BUILD.bazel

@@ -23,6 +23,7 @@ GRPCIO_TESTS_UNIT = [
     "_invocation_defects_test.py",
     "_local_credentials_test.py",
     "_logging_test.py",
+    "_metadata_flags_test.py",
     "_metadata_code_details_test.py",
     "_metadata_test.py",
     # TODO: Issue 16336

src/python/grpcio_tests/tests/unit/_metadata_flags_test.py

@@ -183,7 +183,7 @@ class MetadataFlagsTest(unittest.TestCase):
             fn(channel, wait_for_ready)
             self.fail("The Call should fail")
         except BaseException as e:  # pylint: disable=broad-except
-            self.assertIn('StatusCode.UNAVAILABLE', str(e))
+            self.assertIs(grpc.StatusCode.UNAVAILABLE, e.code())
 
     def test_call_wait_for_ready_default(self):
         for perform_call in _ALL_CALL_CASES: