bectl(8) test: Force destroy the zpool in cleanup

This is a wild guess as to why bectl tests failed once upon a time in CI,
given no apparent way to see a transcript of cleanup routines with Kyua. The
bectl tests construct a new, clean zpool for every test. The failure
indicated was because of a mount that was leftover from a previous test, but
the previous test had succeeded so it's not clear how the mount remained
leftover unless the zpool get health ${pool} had somehow failed.

MFC after: 1 week