MFC r343543: 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.
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