Add support for pytest -n auto

[pauladkisson]

Currently, the multiprocessing inherent in some of the GuPPy workflow steps is preventing parallelized test runs with pytest-n auto. In the CI, sometimes the worker will just hang during the tests when that option is enabled.

To solve this problem, I propose that the tests should run GuPPy with no parallelization, and then there should be dedicated, separate parallel tests to check that parallelizing works properly.