On failure tests-runner should do non-zero exit

On failure tests-runner should do non-zero exit

Right now test runner will always exit(0).
It's helpful to have zfs-tests.sh provide different
exit values depending on if everything passed or not.
We can then use common shell cmds to run tests until failure.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6285