ZTS: Fix zpool_import_hostid_changed_cachefile_unclean_export

Update the test case to freeze the pool then export it to better
simulate a hard failure. This is preferable to copying the vdev
while the pool's imported since with a copy we're not guaranteed
the on-disk state will be consistent. That can in turn result
in a pool import failure and a spurious test failure.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16570