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

gh-135124: Change stdout errors in regrtest worker process (#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.