test: cleanup resources at startup time in system and sample tests

[tswast]

1. Update fixtures to use a consistent prefix, so we can tell which resources are created by python-bigquery (and possibly disambiguate between samples and system tests). Also include a timestamp in the prefix.
2. List all resources in the project, filter by our prefix, delete those resources that were created >= 24 hours ago.

This will prevent resource leaks from getting too bad. Teardown doesn't always get a chance to run because of intermittent Kokoro errors and timeouts.

[tswast]

I experiment with this in #591 (but we'll still need to cleanup the other tests in the samples/snippets directory, even after that is merged.

Datasets are the primary resource that we're concerned about, as they contain tables, models, routines. We should be able to cleanup those before the tests run with a fixture like the following:

RESOURCE_PREFIX = "python_bigquery_samples_snippets"


def resource_prefix() -> str:
    timestamp = datetime.datetime.utcnow().strftime("%Y%m%d_%H%M%S")
    random_string = hex(random.randrange(1000000))[2:]
    return f"{RESOURCE_PREFIX}_{timestamp}_{random_string}"


@pytest.fixture(scope="session", autouse=True)
def cleanup_datasets(bigquery_client: bigquery.Client):
    yesterday = datetime.datetime.utcnow() - datetime.timedelta(days=1)
    for dataset in bigquery_client.list_datasets():
        if (
            dataset.dataset_id.startswith(RESOURCE_PREFIX)
            and dataset.created < yesterday
        ):
            bigquery_client.delete_dataset(
                dataset, delete_contents=True, not_found_ok=True

[tswast]

Also, we could standardize on the prefix format and put the logic here: https://github.com/googleapis/python-test-utils