Update more unittest tests to pytest

[CasperWA]

The filter* tests were still unittests.

This turns them into proper pytests.

[codecov]

Codecov Report

Merging #425 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #425   +/-   ##
=======================================
  Coverage   90.99%   90.99%           
=======================================
  Files          55       55           
  Lines        2522     2522           
=======================================
  Hits         2295     2295           
  Misses        227      227           

Flag	Coverage Δ
#unittests	90.99% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d94d7d...4b81b9c. Read the comment docs.

[ml-evs]

Can't see any problems with this, nice work @CasperWA. I touch a few bits of this in the sorting PR, so we should merge this first and I'll add the extra tests pytest style.