gh-94026: use a temporary file for regrtest worker process stdout

[vstinner]

• Issue: Emscripten test worker sometimes fails witth "BlockingIOError: write could not complete without blocking" #94026

[vstinner]

I create a draft PR since I'm not sure if we should use a file on all platforms or only if Python is run on WASM :-) This change has subtle consequences, stdout file descriptor is inherited by child processes.

[tiran]

#94253 is based on your design, but uses a simpler approach.

[vstinner]

#94253 is based on your design, but uses a simpler approach.

It got merged, I close my draft PR. Thanks @tiran.