[VM/Testing] - Turn on testing in strong NNBD mode.
[VM/Testing] - Turn on testing in strong NNBD mode.

Change-Id: I272de0ca65afb4327d9ec6457c7e514fb1c73449
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139030
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Commit-Queue: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@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