allow user-specified backend for result testrunner

[hoprocker]

[jeffbean]

I think you meant to put the DB backend in the if clause. You changed it to be the CeleryTestSuiteRunnerStoringResult class instead.

if not hasattr(settings, 'CELERY_RESULT_BACKEND') or \
                        settings.CELERY_RESULT_BACKEND not in \
                ['database', 'djcelery.backends.database:DatabaseBackend']:
            settings.CELERY_RESULT_BACKEND = 'database'