gh-133711: Fix test_regrtest for PYTHONUTF8=1 (#134839)

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.