testing: A flake in quickstart_listassets_test: test_list_assets

[tmatsuo]

See:
GoogleCloudPlatform/python-docs-samples#4399

Filtering the query with "bigquery.googleapis.com/Table" should stabilize the test.

[busunkim96]

@tmatsuo This test kept failing when Sijun was trying to merge #58 so I added a filter "iam.googleapis.com/Role" and it seems to be happy now. 😄 https://github.com/googleapis/python-asset/blob/master/samples/snippets/quickstart_listassets_test.py#L26