[Documentation]: How to run tests locally & how to update the test suite

[edwardchalstrey1]

Requested documentation improvement

@tturocy @rahulsavani are there separate tests for the C++ code or does only PyGambit have a test suite?

### Changes required

• There isn't clear documentation on how to run tests locally. In here it says:

  There is a set of test cases in src/pygambit/tests, which can be run
  using nose2.

  • ... however the GH action uses pytest
  • so this should be updated and moved to a new testing section either under contributing or a new page in developer docs; looks like the location of the tests are now tests rather than src/pygambit/tests
• A new section should also explain the current structure of the test suite and how to add new tests. This should cover:
  • Adding new tests for C++ code
  • Adding new tests for Python code in tests

[tturocy]

I have updated the outdated reference to nose in PR #566, and tests are indeed now outside of the package.

There's no separate C++ tests. Our strategy (at least for the moment) is to use the testing in Python.

The test suite doesn't truly have a great structure as yet - we're still evolving this - hence why there's not a great description of it!