Fix TestNg test runner

[lihaoyi]

Fixes #3798, supersedes #3799

Not sure how this ever worked, but it seems like it works now

Converted the example test in #3799 to a unit test.

[lihaoyi]

CC @lefou

[lefou]

Looks good to me.

Tested with a reasonable large TestNG test suite spawning 98 test processes.

Doesn't work properly without testForkGrouping now

[lefou]

@lihaoyi Looks like without custom testForkGrouping, the test suite is now running all tests over and over, probably as often as the count of discovered test classes.

[lihaoyi]

Ah, we must have been running a single task because that task runs all tests within it for some reason

[lihaoyi]

@lefou I pushed an update with a fix and a test case, can try it out if you have time

[lefou]

Looks good. Local tests seem to work for various fork groups. Thank you for this quick fix!

There is now no longer a single test summary at the end but one for each test class, which is a bit less convenient. Maybe Mill should print a summary, which would look more uniform over the various test frameworks anyways.