ZTS: Fix snapshot_009_pos, snapshot_010_pos

Mitigate the likelihood of the newly created volumes being busy
when the 'zfs destroy -r' is issued by waiting for udev to settle.
Since this is not a iron clad fix I've added the test case to
the known list of possible failures and referenced issue #7961.

Finally, in the case this test does fail fix the cleanup logic
so subsequent tests won't incorrectly fail.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7961
Closes #7962