CI broken: pytest raises "fixture applied more than once"

[delgadom]

Tests are failing on master: https://travis-ci.org/github/RhodiumGroup/rhg_compute_tools/jobs/672710341#L2716

This was originally discovered in @dpa9694's PR #65.

Looks like this is related: pytest-dev/pytest#3518

Pytest seems to be wrapping fixtures multiple times when they are declared in testing modules, and recent versions of pytest prevent fixture re-wrapping, and therefore all fixture declarations must be moved to separate (non-testing) modules, e.g. tests/fixtures.py.