Refactor pytest fixtures into a pytest-growler plugin

[lmergner]

I'm copying tests from growler-mako into my extension, so it's probably a good idea to eventually consolidate common fixtures into a pytest plugin at some future point.

[akubera]

Yes, this is on my (long) to-do list for Growler.

[lmergner]

I figured. Is it helpful to leave these ideas as issues?

[akubera]

Yes I believe so. Besides offering a place for discussion, a public list of things that need to be worked on is a good motivator.

What I really need for this issue is a person or guide that can explain best practices for organizing a good pytest module. It looks like I was starting to do a refactor in tests/mock_classes.py, but that was about a year ago. As growler has changed a lot since I started it (>2 years!?! my, how time flies) I haven't really worried about adding support libraries. Since it's starting to solidify and I'm pretty happy with its structure, I will probably look into this (assuming people use it, making it worth my time).