Migrate a VM test off the ancient deprecated unittest package.
Migrate a VM test off the ancient deprecated unittest package.

Change-Id: Id2d408b87bc40af75899fbe0922ba523cbde148c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136022
Auto-Submit: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>
Commit-Queue: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@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