[Test tool] Pass --sound-null-safety to VM unit tests
[Test tool] Pass --sound-null-safety to VM unit tests

This was probably broken since we stopped specifying
--sound-null-safety explicitly in test_matrix.json in
https://dart-review.googlesource.com/c/sdk/+/164160.

TEST=ci

Change-Id: I0a41f13749784db1a076aba434985327291ce71a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212660
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Reviewed-by: Slava Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@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