Temporarily disable testFileContainingExtension

There is a behavioral discrepency in
`DescriptorPool.FindFileContainingSymbol` between the two
implementations of the protobuf python library (one of which is purely
python, and the other utilizes a python extension library written in
C++).

protobuf on PyPI is distributed via various prebuilt wheels, and our CI
setup fetches the relevant one for each platform.  Our Linux instances
end up fetching a pre-compiled C++ version of the library, whereas our
Mac and Windows builders automatically get the pure python version.
This test fails (and our reflection implementation itself) succeeds with
the C++ implementation but fails with the pure python version.

The test will be temporarily disabled pending a fix for
https://github.com/google/protobuf/issues/2882

src/python/grpcio_tests/tests/reflection/_reflection_servicer_test.py

@@ -114,6 +114,9 @@ class ReflectionServicerTest(unittest.TestCase):
                 )),)
         self.assertSequenceEqual(expected_responses, responses)
 
+    @unittest.skip(
+        'TODO(mmx): enable when (pure) python protobuf issue is fixed'
+        '(see https://github.com/google/protobuf/issues/2882)')
     def testFileContainingExtension(self):
         requests = (reflection_pb2.ServerReflectionRequest(
             file_containing_extension=reflection_pb2.ExtensionRequest(