Restore test that passes metadata in a list

The old interface of accepting the metadata as a list, should be kept
due to a backwards incompatibility with a client.

The new ``aio.Metadata()`` type supports iteration, so creating a list
from it, is possible.

src/python/grpcio_tests/tests_aio/unit/metadata_test.py

@@ -213,7 +213,7 @@ class TestMetadata(AioTestBase):
     async def test_from_client_to_server_with_list(self):
         multicallable = self._client.unary_unary(_TEST_CLIENT_TO_SERVER)
         call = multicallable(_REQUEST,
-                             metadata=_INITIAL_METADATA_FROM_CLIENT_TO_SERVER)
+                             metadata=list(_INITIAL_METADATA_FROM_CLIENT_TO_SERVER))
         self.assertEqual(_RESPONSE, await call)
         self.assertEqual(grpc.StatusCode.OK, await call.code())