CLN: use wait_for_job rather than sleep

[jreback]

code here: #24 (comment)
to replace sleep calls in tests.

def wait_for_job(job):

    # from https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/bigquery/cloud-client/snippets.py

    while True:
        job.reload()  # Refreshes the state via a GET request.
        if job.state == 'DONE':
            if job.error_result:
                raise RuntimeError(job.errors)
            return
        logger.info("Waiting for {} to complete".format(job))
        time.sleep(1)

[jreback]

cc @MaximilianR
cc @mr-mcox

[max-sixty]

And the logging is at the author's discretion; probably not needed for tests

[tswast]

The latest version uses .result() on the job object to do something similar to wait for the job to finish.