[infra] Fix test runner error when force closing subprocess stdio
[infra] Fix test runner error when force closing subprocess stdio

The code tried to create the command output object twice in this case.

Change-Id: Icfb413ba329738cb732330d22885f62ccb7a99e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150102
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: 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