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

sbin/bectl/tests/bectl_test.sh

@@ -62,7 +62,7 @@ bectl_cleanup()
 	zpool=$1
 
 	if zpool get health ${zpool} >/dev/null 2>&1; then
-		zpool destroy ${zpool}
+		zpool destroy -f ${zpool}
 	fi
 }