RF: Use Mixin class for extraction tests

[scottclowe]

When we have tests which are common across multiple objects to test, a handy way of doing this is to make multiple subclasses with different setup but the same test methods. When making the parent subclass, we want it to contain a list of all the tests to run and no set up, and not actually get run itself. We can't subclass BaseTestCase directly because then it is a subclass of unittest.TestCase and will automatically be collected by pytest. Our solution is to use multiple inheritance with a Mixin class. However, the previous way of doing this was not quite doing Mixin classes correctly and it wasn't triggering both parent's __init__ methods. This resolves the situation into a correct Mixin class. c.f. https://stackoverflow.com/a/50465583/1960959

[codecov]

Codecov Report

Merging #191 (20867d9) into master (e95420b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
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  Files           8        8           
  Lines         911      911           
  Branches      188      188           
=======================================
  Hits          848      848           
  Misses         32       32           
  Partials       31       31           

Flag	Coverage Δ
unittests	92.97% <ø> (ø)

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