[3.13] gh-133711: Fix test_regrtest for PYTHONUTF8=1 (GH-134839)

[miss-islington]

Use "backslashreplace" error handler to decode stdout and stderr.
Example:

vstinner@WIN C:\victor\python\main\build\test_python_worker_8360\x91> "C:\victor\python\main\PCbuild\amd64\python_d.exe" -m test --fast-ci --slow-ci --testdir C:\Users\vstinner\AppData\Local\Temp\tmp0t59e8da test_regrtest_noop1 test_regrtest_noop2 test_regrtest_noop3 test_regrtest_noop4 

Notice the "\x91" byte at the end of the first line: it's the
non-ASCII U+00E6 character encoded to the OEM cp437 code page.
(cherry picked from commit 9161827)

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

• Issue: Enable UTF-8 mode by default (implement PEP 686) #133711