Improve test coverage

[bjwatson]

What:

Improve test coverage in gax-python. Areas of inadequate coverage are detailed in reports like https://codecov.io/gh/googleapis/gax-python/tree/d59635e37d2420908a2828da8f54c0bbbd4cf4e6/google/gax

Why:

We already have 97.2% coverage, which is very good, but we can challenge ourselves to achieve 99%+ coverage as a stretch goal.

[theacodes]

@lukesneeringer how would you feel about switching to py.test style tests ala google-auth? If we ever want to, now is the time to do it while this library is relatively small.

I would be willing to help with that switch and getting us to 100% coverage.

[lukesneeringer]

Since @bjwatson wrote that ticket, I think coverage had dropped. It was ~80% when I started.

This is a pretty high priority for me as soon as I finish the CI on the manual layers, and I will switch over to py.test.

[theacodes]

Cool, I'm happy to help with code review or anything else related to that, and I'm happy to help update the test code I wrote. :)

[lukesneeringer]

I will loop you in as soon as I can pivot there. I want to do something similar on gax that I am doing on google-cloud-python, but I need to finish it on one repo at a time. :-)

And google-cloud-python is stuck in "get it working in AppVeyor" hell still...

[theacodes]

I want to do something similar on gax that I am doing on google-cloud-python

What did you have in mind (other than adding appveyor tests)? I recently overhauled testing in #161