gh-109566: regrtest reexecutes the process (#109909)

When --fast-ci or --slow-ci option is used, regrtest now replaces the
current process with a new process to add "-u -W default -bb -E"
options to Python.

Changes:

* PCbuild/rt.bat and Tools/scripts/run_tests.py no longer need to add
  "-u -W default -bb -E" options to Python: it's now done by
  regrtest.
* Fix Tools/scripts/run_tests.py: flush stdout before replacing the
  process. Previously, buffered messages were lost.