Revisit DuplicatedTestJob exception

[chaws]

SQUAD will not fetch a TestJob which job_id is already present in any TestRun of the Build. While this was the case for early days of SQUAD, that might not be the scenario anymore. Now we have multiple backends and occasionally it could happen that the same Build sent two TestJobs to two different backends and they return the same exact job_id in LAVA. It's very, very, very, very unlikely, but it could happen:

            job_id = metadata_fields.get('job_id')
            if job_id is None:
                metadata_fields['job_id'] = uuid.uuid4()
            elif build.test_runs.filter(job_id=job_id).exists():
                raise exceptions.DuplicatedTestJob("There is already a test run with job_id %s" % job_id)