Improve testing suite

[daviesje]

A few issues with the automated tests should be addressed.

A short/incomplete wishlist of extra tests:

• tests of spin-temperature calculations i.e how a particular radiative background effects Ts, Tk, x_e
• tests of mass function integrals / values, perhaps against hmf.
• Performance tests on a few simple cases to ensure nothing has slowed down.

We also want to rein in the time taken on the tests, some interpolation tables and example runs (usually the minihalo ones) run very slowly, and this can impact development

[daviesje]

There is also the matter of github workflow tests sometimes failing due to not being able to find a particular file or folder