Port testsuite to pytest, using sphinx's testing module

[vermeeren]

Current unit tests are rather hacky, Sphinx and Breathe keep diverging due to fundamental differences in used frameworks. Over time, more small issues are likely to show up that need even more hacks to keep things working.

Porting the testsuite to pytest and using sphinx's testing module will simplify Breathe's testing itself and make it possible to use Sphinx's own testing fixtures, among other things. This will reduce the complexity of our own tests as we let Sphinx handle more and more things for us.

See #472 for further details, Sphinx doc is at https://www.sphinx-doc.org/en/master/devguide.html#unit-testing.

[vermeeren]

Will be fixed in #487.