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