Add --prioritize to regrtest

[ambv]

From the added help:

--prioritize TEST1,TEST2,...
                 select these tests first, even if the order is randomized.

--prioritize is used to influence the order of selected tests, such that
the tests listed as an argument are executed first. This is especially
useful when combined with -j and -r to pin the longest-running tests
to start at the beginning of a test run. Pass --prioritize=test_a,test_b
to make test_a run first, followed by test_b, and then the other tests.
If test_a wasn't selected for execution by regular means, --prioritize will
not make it execute.

This is useful in general, but in particular for bigmem tests on buildbots where test_bigmem, test_lzma, and test_bz2 can take over 30 minutes each. Guaranteeing that the test run starts with those shortens the overall runtime of tests, which is otherwise pretty unpredictable due to -r.

Linked PRs

• gh-132678: Add --prioritize to regrtest #132679

[ambv]

--start-with would be a slightly better name, but there's already --start that means something else entirely, so I opted for --prioritize, which is clear as well and very different from --start.

[ambv]

We won't be backporting this.