[nnbd] Make PRESUBMIT.py warn on upload if tests are not changed in NNBD and non-NNBD suites
[nnbd] Make PRESUBMIT.py warn on upload if tests are not changed in NNBD and non-NNBD suites

This CL ensures that the presubmit check will complain if newly added
tests were only added to NNBD suite but not to non-NNBD (or vice versa).

Also add ffi test suites to the list of checked directories.

Change-Id: I090064b99e5ca30444ab53edf1f262572f60beb9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149426
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@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