BigQuery: fix IPython system test for new pytest (#5400)
BigQuery: fix IPython system test for new pytest (#5400)

* BigQuery: fix IPython system test for new pytest

Pytest changes the way that marks and fixtures work in pytest 3.6.0 that
breaks when using unittest-style tests. Use pytest-style test for the
IPython system test so that the fixture gets used.

* flake8 fix

googleapis/python-bigquery,googleapis/python-bigquery