Bug fixes for single test runs in zfs-tests

Correctly remove the temporary runfile after the
single test is run.

Cleanup and setup scripts are relative to the
test suite's location, correct how we look for
those scripts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5844