Test related to-many behavior

[rpkilby]

Add tests for filtering across to-many relationships. Includes:

• A few tests for filtering across to-many relationships w/ filtersets. Note that these are currently expected failures, and will be fixed with the related filtering rework.
• Tests that describe assumptions about the behavior of filtering across to-many relationships. These are used to help validate the expected behavior of filtersets.

ref #217

[codecov-io]

Codecov Report

Merging #219 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #219   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files           4        4           
  Lines         248      248           
  Branches       53       53           
=======================================
  Hits          243      243           
  Misses          1        1           
  Partials        4        4

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[rpkilby]

The tests can probably be improved, but I think it's Good Enough^TM for now.