Refactoring test fixtures

[tturocy]

Trying to understand the output from the failing Nash equilibrium tests recently just about did my head in, so I decided it was high time that we really need to look at our tests. So I pulled together a bunch of notes/thoughts I had been having (alongside reading up on some pytest features)..

This draft pull request is a proposal for discussion.

In short - I have re-written what used to be the test for enummixed_solve with rational probabilities to be a generic Nash equilibrium solver tester (!!!)

Highlights:

• Test cases are represented as a dataclass
• Games are constructed using a factory function (this solves a problem we noted previously that if the game construction fails it takes down the test run at collection time)
• The solver is likewise specified as a callable
• Tests are given an identifier of our choosing. I have been lazy and just called them test1, test2 &c right now, they will need to be better in production of course.
• Tolerances are configurable for regrets and probabilities on a per-test case. So no global setting is hard-coded, and we can handle both float and rational with the same test function.
• The tests now use pytest-subtests (which has been added as a dependency in the tests requirements.txt). This allows us to isolate the individual checks within the test so even if one fails the others can still run - e.g. if somehow max_regret is correct but the probabilities change for one profile, it still will check them all.
• Experimenting with using Q() as a shorthand for rationals just to make our intent precise, and a helper d() which visually represents a probability distribution - solely for visual layout so we are not lost in a maze of square brackets, all alike.

This is an all hands request for everyone to have a look. Not least because in the new year I will be asking everyone to kick in for rationalising our test suite using either this technique or some other one - it is clear our test suite's reaching the edge of maintainability without making some investments like the ones suggested here!