Allow zfs-tests to run a single test

Add a -t flag to zfs-tests to allow a user
to run a single test by providing the path
to the test relative to STF_SUITE.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5775