Make the Azure CI runtests testing run using multiprocessing.

[stuartarchibald]

As title, aim is to speed up the runtests check.

[stuartarchibald]

CI is failing, this is because the runtests test isn't using Numba's test runner but rather running the test module itself directly through it's __main__ (hence flags like -m don't work). Needs discussion.

[stuartarchibald]

Conclusion from discussion at the maintainer meeting was that the "test runtests" part of the CI testing can be massively shortened to essentially just check that:

• the test suite reports it is at least X tests in size
• that at least one thing can be compiled and executed correctly.

This should reduce the "test runtests" run time from ~90s to probably nearer 10s.

[stuartarchibald]

Closing, replaced by #8927, which implements #8918 (comment).