Add zfs-test facility to automatically rerun failing tests

This was a project proposed as part of the Quality theme for the
hackthon for the 2021 OpenZFS Developer Summit. The idea is to improve
the usability of the automated tests that get run when a PR is created
by having failing tests automatically rerun in order to make flaky
tests less impactful.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #12740