BigQuery: Move maybe_fail_import() to top level test utils

[plamut]

Following a recent discussion, this PR moves a test utility from BigQuery to the top-level test utils, since it is considered useful outside of BigQuery, too.

How to test

Primarily verify that the promoted utility function is general enough for the promotion.

[tseaver]

FWIW, after two years, there are exactly zero uses of maybe_fail_import outside of the python-bigquery repo.

[plamut]

Sounds like we need to run a large-scale AdWords campaign to promote its usage.

@tseaver Are you suggesting that it might be better to move this back to the python-bigquery repo?

[tseaver]

@plamut

Are you suggesting that it might be better to move this back to the python-bigquery repo?

I'm not sure. I came across it while working on googleapis/python-test-utils#43: the function has no test coverage, which meant that I had to install a development version into a virtualenv along with python-bigquery so that I could test that I didn't break anything.

[plamut]

@tseaver I see. Sounds like a one-off (ish) annoyance, thus might not be worth it. We could cover it with tests, however, I'll create a ticket to track it.

Edit: Created.