debian: Re-run failed unit tests once.

There are always a few unit tests that have some race conditions
that cause them to occasionally fail. It is probably a good idea
to re-run the failed tests once while building deb packages.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Co-authored-by: Keith Amidon <kamidon@nicira.com>
Signed-off-by: Keith Amidon <kamidon@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>