[3.11] gh-115421: List all test/ subdirs in Makefile, and test them (GH-115813)

This backports:
- GH-115813
- GH-115422

Unlike on the main branch, new directories are added to the end,
so they're a bit easier to patch out if a redistributor needs to do so.

On main & 3.12, there's a special case for `idlelib/idle_test`; on
3.11 TESTSUBDIRS has several more entries that are not in `test/`.
This backport ignores all of them (including idlelib).
(The alternative would be list them, as additions to TEST_HOME_DIR.
But that's probably too invasive; people might split stdlib up in
surprising ways.)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>