grunt: Also delete QUnit temp files when test fails

Move post-qunit task to qunit.done event which fires both on
success and failure.

Define pre-qunit + qunit as a group test 'unit', so they can
be run from the command line (previously 'grunt qunit' would
fail) as 'grunt unit'.

Also add override comment to css file using non-standard
property so csslint passes again (follows-up b2fbe35).

Bug: 49431
Change-Id: I5079d00a63d43276a12dd78c306bb3819470631d
Merge "grunt: Also delete QUnit temp files when test fails"