gh-109276: libregrtest: WASM use filename for JSON

[vstinner]

On Emscripten and WASI platforms, or if --python command line option is used, libregrtest now uses a filename for the JSON file.

Emscripten and WASI buildbot workers run the main test process with a different Python (Linux) which spawns Emscripten/WASI processes using the command specified in --python command line option. Passing a file descriptor from the parent process to the child process doesn't work in this case.

• Add RunTests.json_file_use_filename() method.
• Add a test in test_regrtest on the --python command line option.
• test_regrtest: add parallel=False parameter.

• Issue: Enhance Python regrtest test runner (test.libregrtest) #109276

[vstinner]

First, I wrote PR #109326 to debug the issue and understand what's going on. Then I wrote this clean PR.

Well. Sadly, this PR doesn't work as expected because I misunderstood how Emscripten/WASI buildbot workers run tests.

On these buildbots, a first Linux x86-64 Python binary is used to run python -m test. This one lists tests and then spawns test worker processes.

These test worker processes use the Emscripten/WASI runtime! So the parent and child processes don't run the same "platform"!

Before, the result was serialized as JSON at the end of stdout. But it causes issues, so I enhanced libregrtest to write the JSON into a separated file descriptor (change made 2 days ago).

First probem: apparently, currently, it's not possible to pass a file descriptor from the Linux x86-64 Python to the Emscripten/WASI Python: it fails with OSError: [Errno 8] Bad file descriptor.

So I wrote this PR to use a named filename for the JSON file. Second problem: the Emscripten/WASI Python runtime doesn't have a full access to the filesystem, and paths are mapped differently.