login test [nfc]: Add await for handlePlatformMessage

The later `await tester.idle` call helps wait for the earlier
mock platform message to be handled.  We can remove that call and use
await on the handlePlatformMessage call itself.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

test/widgets/login_test.dart

@@ -219,10 +219,9 @@ void main() {
 
         final ByteData message = const JSONMethodCodec().encodeMethodCall(
           MethodCall('pushRouteInformation', {'location': url.toString()}));
-        tester.binding.defaultBinaryMessenger.handlePlatformMessage(
+        await tester.binding.defaultBinaryMessenger.handlePlatformMessage(
           'flutter/navigation', message, null);
 
-        await tester.idle();
         check(testBinding.takeCloseInAppWebViewCallCount()).equals(1);
 
         final account = testBinding.globalStore.accounts.single;