Tox runs tests via unit_tests.sh which when not
running a test subset will mean tox only checks for
the final exit code. This means horizon test failues
will not be seen by tox unless openstack_dashboard
tests also fail.
This change forces an exit code which !=0 if either
test run fails.
Change-Id: I8fbcb834d9817e7eea1b2c39fc4caab6004b981b