> About https://bugs.python.org/issue40275#msg369214, shall we continue to improve the libregretest?

If someone wants to continue the effort in libregrtest, I suggest to open a new issue. This one already has a long list of commits and messages about test.support.

See test.libregrtest.runtest_mp module for the "multiprocessing" implementation which uses subprocess. It's used when you run tests using -jN option. Almost all buildbots use -jN: see make buildbottest.