Test runner feature request: capture stderr

Many test runners, including unittest (via `--buffer` option), allow to capture stderr and show it only as part of the failure in the end.

It would be great to implement this option in the parallel test runner, as it would allow to see all the relevant error information in the end of the runner logs (e.g. on CI) instead of hunting for the stderr output somewhere in the middle of the log. cc @sbc100