Add script to run and automatically retry tests (#542)

* Add script to run and automatically retry tests

* Review feedback and other minor improvements

- keep-failed-namespaces has been changed to delete-failed-namespaces
  (default false)
- A unique work directory is created for each test run to avoid
  interference
- The logs now contain the exact command that was used to run the tests
- Script tried to delete already deleted namespaces

Author: lfrancke