ZTS: Fix vdev_zaps_004_pos.ksh
When attaching a vdev to a mirror wait for the resilver to complete
before invoking zdb to inspect the pool. This ensures the pool is
essentially idle which allows zdb to open the imported pool reliably.
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13112
Closes #6935