Add delay between zpool add zvol and zpool destroy

As investigated by #14026, the zpool_add_004_pos can reliably hang if 
the timing is not right. This is caused by a race condition between 
zed doing zpool reopen (due to the zvol being added to the zpool), 
and the command zpool destroy.

This change adds a delay between zpool add zvol and zpool destroy to
avoid these issue, but does not address the underlying problem.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Issue #14026
Closes #14052

tests/zfs-tests/tests/functional/cli_root/zpool_add/zpool_add_004_pos.ksh

@@ -74,4 +74,8 @@ log_must zpool add $TESTPOOL $ZVOL_DEVDIR/$TESTPOOL1/$TESTVOL
 
 log_must vdevs_in_pool "$TESTPOOL" "$ZVOL_DEVDIR/$TESTPOOL1/$TESTVOL"
 
+# Give zed a chance to finish processing the event, otherwise
+# a race condition can lead to stuck "zpool destroy $TESTPOOL"
+sleep 1
+
 log_pass "'zpool add <pool> <vdev> ...' adds zfs volume to the pool successfully"