[3.11] [3.12] gh-111165: Move test running code from test.support to libregrtest (GH-111166) (GH-111316) (GH-111318)

Remove no longer used functions run_unittest() and run_doctest() from
the test.support module.
(cherry picked from commit f6a45a03d0e0ef6b00c45a0de9a606b1d23cbd2f)
(cherry picked from commit 5c4f9a1c7ed9fddd8c3ce4c1793af07f66f36a6b)