msglist test [nfc]: Cut "async" from main function

This isn't used.  For a test file, it'd be inappropriate if it were,
because the only thing `main` itself should do is to register tests,
not run any test or app code.

Author: gnprice

test/model/message_list_test.dart

@@ -20,7 +20,7 @@ import '../stdlib_checks.dart';
 import 'content_checks.dart';
 import 'test_store.dart';
 
-void main() async {
+void main() {
   // These variables are the common state operated on by each test.
   // Each test case calls [prepare] to initialize them.
   late Subscription subscription;