[3.13] gh-135124: Change stdout errors in regrtest worker process (GH-135138) (#135169)

[3.13] gh-135124: Change stdout errors in regrtest worker process (GH-135138) (#135169)

gh-135124: Change stdout errors in regrtest worker process (GH-135138)

Set sys.stdout encoder error handler to backslashreplace in regrtest
workers to avoid UnicodeEncodeError when printing a traceback
or any other non-encodable character.

Move the code from the Regrtest class to setup_process().

Call setup_process() earlier, before displaying regrtest headers.
(cherry picked from commit 3d396ab7591d544ac8bc1fb49615b4e867ca1c83)

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