Test fixture usage tool

[davidorme]

A while back, I wrote a short script to profile the fixtures in the pytest suite and then generate some simple stats and a graph representation of fixture usage by tests. The aim here is to allow us to identify potential duplicate fixture functionality and lower the maintenance load on the test suite.

We might never use it again, but it's sitting in my local clone at the moment and seems worth hanging on to in case we do want it again, even if it is just to count the tests...