[3.12] gh-101634: regrtest reports decoding error as failed test (GH-106169) (#106174)

gh-101634: regrtest reports decoding error as failed test (GH-106169)

When running the Python test suite with -jN option, if a worker stdout
cannot be decoded from the locale encoding report a failed testn so the
exitcode is non-zero.
(cherry picked from commit 2ac3eec103cf450aaaebeb932e51155d2e7fb37b)

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