zts: allow running a single test by name only

Specifying a single test is kind of a hassle, because the full relative
path under the test suite dir has to be included, but it's not always
clear what that path even is.

This change allows -t to take the name of a single test instead of a
full path. If the value has no / characters, we search for a file of
that name under the test root, and if found, use that as the full test
path instead.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16088