[samples/ffi] Run the async samples in sequence

The log prints are not very useful when running multiple similar tests
in paralel.
This CL changes the async tests to be run in sequence.

Bug: https://github.com/dart-lang/sdk/issues/49361
Change-Id: I73feb673c770be366cb0c6d2e80379dff53da987
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250086
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Auto-Submit: Daco Harkes <dacoharkes@google.com>

samples/ffi/async/async_test.dart

@@ -9,7 +9,10 @@
 import 'sample_async_callback.dart' as sample0;
 import 'sample_native_port_call.dart' as sample1;
 
-main() {
-  sample0.main();
-  sample1.main();
+main() async {
+  print('==========');
+  await sample0.main();
+  print('==========');
+  await sample1.main();
+  print('==========');
 }