Test the c-ares resovler on Jenkins, should be removed before submission

tools/run_tests/run_tests.py

@@ -254,7 +254,8 @@ class CLanguage(object):
                  _ROOT + '/src/core/tsi/test_creds/ca.pem',
              'GRPC_POLL_STRATEGY': polling_strategy,
              'GRPC_VERBOSITY': 'DEBUG'}
-        resolver = os.environ.get('GRPC_DNS_RESOLVER', None);
+        # TODO(zyc): change this back before submission
+        resolver = 'ares';
         if resolver:
           env['GRPC_DNS_RESOLVER'] = resolver
         shortname_ext = '' if polling_strategy=='all' else ' GRPC_POLL_STRATEGY=%s' % polling_strategy