Create special pytest fixtures to facilitate component-specific testing of lifecycle functions

[breezykermo]

It would be great to be able to be able to test analysers (and maybe selectors as well) by running them singularly on test elements.

These could be made available as pytest fixtures, being setup in src/conftest.py. Ideally, what would be passed as a fixture would be something like higher level function: get_analyse_element, to which could be passed the class for the analyser that is being tested. Using the get_etype static methods, the fixture could work out what kind of element should work with the analyser.

After calling get_analyse_element, the returned object could have some kind of preloaded test_analyse_element attribute, which would run the module's analyse_element for a range of input elements, checking that appropriate errors are thrown in the case of incompatible input elements, and verifying things on the output elements in some way.