ZTS: Cleanup some cleanup functions

ZTS: Cleanup some cleanup functions

Cleanup functions should make a best effort to clean up as much as
possible.

Do a consistency pass in a bunch of tests to make the cleanup
functions less prone to failure and fix a few typos here and there.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9993