Fix warnings raised during testing.

[ngnpope]

Django raises RuntimeWarning when duplicate models are registered. By declaring the models inside a test function which uses @pytest.mark.parametrize(...), many messages can be triggered. These generate noise in the pytest summary.

I wasn't sure whether it was more desirable to keep the models close to their respective test function or group them at the top. I went for the former...

[codecov]

Codecov Report

Merging #522 (6b79874) into master (93935c7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   98.62%   98.62%           
=======================================
  Files          58       58           
  Lines        5951     5951           
=======================================
  Hits         5869     5869           
  Misses         82       82           

Impacted Files	Coverage Δ
tests/test_regressions.py	100.00% <100.00%> (ø)
tests/test_versioning.py	100.00% <100.00%> (ø)

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[tfranzel]

even though i don't like removing the models from the test scope for completeness, these are warnings nonetheless. the tests also serve as documentation and thus should be as isolated and compact as possible.

added a docstring with the test name to make that abundantly clear.