[dart2js] Clean up multitest structure in async_await_js_transform_test.

These multitest comments interact poorly with `dart format` and make it
difficult to discern what's actually happening. This rewrite should be
clearer.

Change-Id: If5ad8c61be8d311fd9d1450897eeb88ec799e477
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427585
Reviewed-by: Nate Biggs <[email protected]>

Author: fishythefish