cpython: include test.support and test.regrtest

test.{support, regrtest} are the internal packages cpython
developers use to write tests.
Although they are not public and the API may change/break
some developers use these packages to write tests for their
(3rd party) software.
The derivations for cpython now only remove the actual tests
but leave the packages in place that are used to write them.

Discussion: https://github.com/NixOS/nixpkgs/pull/28540