MFC r319800,r319806:

r319800:

Don't explicitly get the class to PART in gctl_test_helper.c

This will allow the tool to be used with arbitrary geom(4) classes, like GEOM.

Specify class=PART explicitly in the tester to keep existing behavior.

r319806:

Improve handling with system state

• Always unlink $cmd after exit via END block.
• The tests don't function well if kern.geom.debugflags != 0. Save debugflags, then restore them at the end of the test.