BRT: Fix tests to work on non-empty pools

It should not normally happen, but if it does, better to not fail
everything for no good reason, or it may be hard to debug.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16007