Issue 37873: unittest: execute tests in parallel

The unittest documentation makes reference to a potential parallelization feature:

"Note that shared fixtures do not play well with [potential] features like test parallelization and they break test isolation. They should be used with care." (https://docs.python.org/3/library/unittest.html)

However, it seems that executing tests in parallel is not yet a feature of unittest. This enhancement request is to add parallel execution of tests to unittest.

A command line option may be a good interface. Ideally, it would be compatible with test discovery. Outside of the Python ecosystem, a common practice is to define test cases in a Makefile and then execute GNU Make with the '-j' flag (https://www.gnu.org/software/make/manual/html_node/Parallel.html#Parallel). Adding such an option to unittest would be a convenience and may save the effort of bringing in additional libraries or tools for parallel unit test execution.

test.regrtest has a  -j option. Perhaps some of the Python coding for that could be used for unitest also.

See also https://mail.python.org/pipermail/python-ideas/2017-September/047100.html . One of the ideas in the thread was to move test.regrtest parallel execution functionality into unittest. I think this would be good to have it in unittest like support in pytest for -j.

Leveraging GNU Parallel (https://www.gnu.org/software/parallel/) might help simplify implementation. Perhaps that could be used as a subprocess call?

> Leveraging GNU Parallel (https://www.gnu.org/software/parallel/) might help simplify implementation. Perhaps that could be used as a subprocess call?

In general, we attempt to avoid depending on the availability of external tool. For example, I don't expect this tool to be available on Windows, whereas it would be better to support parallel execution on Windows as well.