gh-84461: Add ability for multiprocessed libregrtest to use a different Python executable

[emmatyping]

wasm32-emscripten does not support subprocesses. As a workaround, we can run the regression test suite with e.g. the system-native Python used for cross building, then execute the actual subprocess workers with the emscripten/target runtime.

For example, consider a setup where you have two folders: linux, which includes a build of CPython for linux and the native architecture of the machine. You also have wasm, which is the cross compiled Python for emscripten/node.

This PR would enable you to do linux/python -m test -j1 -p "node wasm/python.js", to run the test suite workers in nodejs.

[brettcannon]

@ethanhs can you add a news file?

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

Sorry, I can't merge this PR. Reason: 4 of 7 required status checks are expected..

[miss-islington]

@ethanhs: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: Head branch was modified. Review and try the merge again..