Partest should audit flags and pending tests

[scabug]

Partest needs an -audit mode to report when pending tests become runnable and when test flags become superfluous.

Possibly, it could try arbitrary permutations of flags and report that -Xfoo doesn't affect the outcome.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8867?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.2

[som-snytt]

That's an interesting idea, to minimize flags, except for example -Wunused to ensure no false positives is not minimal. Or, like some neg tests, it would require a "dummy error" that would be a true positive, in order to fail the test, and the check ensures only true positives are reported.