Test configuration for all tests in directory does not allow debug mode

Test scope is 'Test in file, filtered by name':

Test scope is 'All in directory':

## More context on this issue
Why this happens
- Using a test scope of directory means that running tests will be done through `DefaultRunProgramRunner` instead of `FlutterTestRunner`
- This is because `FlutterTestRunner` returns false for directory tests for either run mode (run or debug)
- There's no default runner for debugging, so that's why the debug button doesn't show up

What we'd like to have ideally
- Both run and debug run through `FlutterTestRunner` (or other custom runner) so that we can use `--start-paused` and have debugging for directory tests

Implementation steps
- Manage multiple observatory connections for one process handler (see `FlutterTestRunner::runInDebugger` and `Connector` class)
- Similarly, allow multiple `XDebugProcess`s for an `XDebugSession`