fix test mode running under rails3

We don't want to load the test-unit gem, and we also need to tell rails
that we're using rspec not test-unit.

Change-Id: Ia8d9ef1e8adaab5a4c55c1324cca541a664c1b02
Reviewed-on: https://gerrit.instructure.com/31742
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>