Add missing reply in C++ MethodChannel unit test (#20420)

The test handler wasn't replying, which logged an error message during
the unit test.

Fixes flutter/flutter#62560

Author: stuartmorgan-g

shell/platform/common/cpp/client_wrapper/method_channel_unittests.cc

@@ -67,6 +67,7 @@ TEST(MethodChannelTest, Registration) {
         // result.
         EXPECT_EQ(call.method_name(), method_name);
         EXPECT_NE(result, nullptr);
+        result->Success();
       });
   EXPECT_EQ(messenger.last_message_handler_channel(), channel_name);
   EXPECT_NE(messenger.last_message_handler(), nullptr);