gh-108388: regrtest splits test_asyncio package (#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.