Fix race condition in hotspare_scrub_002_pos

verify_assertion: Don't assert that $ZPOOL scrub exits 0.

The pool may have already started resilvering, and in this test the
postconditions are strict enough to enforce that the results meet
expectations, i.e. at least one error was detected, which can only be the
case if the pool scrubs or resilvers.

Submitted by: Will
Sponsored by: Spectra Logic Corp