Support fast "run tests" when the test set only includes/excludes full files

[DanTup]

When running "Run All Tests" with noDebug from the VS Code runner, we will do the fast thing (run dart test test instead of sequentially running each file) only if you're running the whole set.

However, it should be possible to do the same if the selection is a complete set of files (eg. all included TestItems are suites) and any exclusions are also complete files (or groups/tests that are not in the included set).

[DanTup]

This requires being able to pass multiple files to the DA (whereas right now we pass a single program that is the test folder).

Some comments/ideas on https://github.com/Dart-Code/Dart-Code/tree/faster-multiple-suites branch.