* Quiet debug test runner output when in machine mode.

See: https://github.com/flutter/flutter-intellij/issues/1176

* json => machine

This adds a way to run only a subset of the tests.
(The new flags do the same thing as 'pub run test'.)

(The Flutter plugin will use this to update the UI with test progress.)

- remove unused collector parameter
- clarify a comment
- inline _currentPackageTestDir

And add comments explaining why.

Currently this just prints the observatory URL as a JSON event.
Refactored the code to make this fit in.