[3.12] gh-108388: regrtest splits test_asyncio package (GH-108393) (#108397)

gh-108388: regrtest splits test_asyncio package (GH-108393)

Currently, test_asyncio package is only splitted into sub-tests when
using command "./python -m test". With this change, it's also
splitted when passing it on the command line:
"./python -m test test_asyncio".

Remove the concept of "STDTESTS". Python is now mature enough to not
have to bother with that anymore. Removing STDTESTS simplify the
code.
(cherry picked from commit 174e9da0836844a2138cc8915dd305cb2cd7a583)

Co-authored-by: Victor Stinner <vstinner@python.org>