Dart-code: Support running all tests whilst debugging

[tomgilder]

I don't know if this is a VS Code or Dart Code issue, but currently you can't access the "Run All Tests" command whilst debugging an app.

You can still run individual tests, so I don't see why you can't run them all. I sometimes want to check all tests are passing without existing the debugging session.

[DanTup]

I don't think there's a great reason to prevent this (it was done to avoid you hitting the button twice and spawning two lots of them all). I've removed the restriction and confirmed I can run tests while an app is running. Thanks for the suggestion!