celery: test case helper

• Adds testcase class helper for easier testing of Celery tasks. The helper will create a new Celery application per test, similar to the way InvenioTestCase creates a Flask application per test.

• Adds example test case using the new helper.

• Removes no longer needed test runner hack.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>