gh-109276, gh-109508: Fix libregrtest stdout (#109903)

gh-109276, gh-109508: Fix libregrtest stdout (#109903)

Remove replace_stdout(): call sys.stdout.reconfigure() instead of set
the error handler to backslashreplace.

display_header() logs an empty line and flush stdout.

Remove encoding workaround in display_header() since stdout error
handler is now set to backslashreplace earlier.