Test QueryDict against CSV filters

[rpkilby]

Verify expected behavior from #935. Essentially, the CSV filters should treat:

MyAPI/mymodel?mysubmodel__in=1&mysubmodel__in=5&mysubmodel__in=9

as equivalent to:

MyAPI/mymodel?mysubmodel__in=9

• Refactors tests to use Python 3's subTest feature.
• Updates tests to test against expected IDs instead of just a count.
• Adds a QueryDict case to test the above expected behavior.

[codecov-io]

Codecov Report

Merging #937 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #937   +/-   ##
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  Coverage   97.95%   97.95%           
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  Files          15       15           
  Lines        1122     1122           
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  Hits         1099     1099           
  Misses         23       23

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[carltongibson]

This is fine.

Why do they take the last value (rather than the first say?)

(I have no issue with it either way: it's not the intended usage for CSV widgets...)

[rpkilby]

Why do they take the last value (rather than the first say?)

I honestly have no idea.