bpo-45410: Enhance libregrtest -W/--verbose3 option (GH-28908)

bpo-45410: Enhance libregrtest -W/--verbose3 option (GH-28908)

libregrtest -W/--verbose3 now also replace sys.__stdout__,
sys.__stderr__, and stdout and stderr file descriptors (fd 1 and fd
2).

support.print_warning() messages are now logged in the expected
order.

The "./python -m test test_eintr -W" command no longer logs into
stdout if the test pass.