[ CLI ] Avoid awaiting on join() calls in test harness
[ CLI ] Avoid awaiting on join() calls in test harness

Fixes #47750

Change-Id: I179a9e987cea157257d40d6e840e9f4ff5539877
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221543
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk