tests: add docker-test.sh --ref giant|firefly etc.

Add the --ref option to chose the git ref to which the working tree must
be reset instead of HEAD.

   test/docker-test.sh --ref giant make

is equivalent to

   test/docker-test.sh bash -c 'git reset --hard giant ; make'

Signed-off-by: Loic Dachary <ldachary@redhat.com>