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

[helin24]

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 XDebugProcesss for an XDebugSession