Filter verification tests

[NoraLoose]

This PR adds verification tests for the filter algorithm, in preparation for the refactor in #149, which replaces the filter algorithm with a new one.

This PR essentially mirrors #79, but here the verification tests are associated with the test_filter.py module (rather than the test_kernels.py module as in #79). The main changes are:

• I moved fixture definitions from tests/test_filter.py to tests/conftest.py and, in the process, got rid off a lot of redundant code.
• I added the actual verification tests via test_filter_validation.py.

[NoraLoose]

@rabernat do you agree that we can merge this PR?

[rabernat]

Thanks for the ping Nora! Yes, absolutely, I will do it now.