message: fix building tests with gcc
  Add mocks to MockGrpcMuxWatch. follow-up to
diff: @@ -96,8 +96,11 @@ public:
  MOCK_METHOD(void, start, ());
  MOCK_METHOD(void, pause, (const std::string& type_url));
  + MOCK_METHOD(void, pause, (const std::vector<std::string> type_urls));
  MOCK_METHOD(void, resume, (const std::string& type_url));
  + MOCK_METHOD(void, resume, (const std::vector<std::string> type_urls));
  MOCK_METHOD(bool, paused, (const std::string& type_url), (const));
  + MOCK_METHOD(bool, paused, (const std::vector<std::string> type_urls), (const));
  MOCK_METHOD(void, addSubscription,
  (const std::set<std::string>& resources, const std::string& type_url,
  