gh-109276: libregrtest: WASM use filename for JSON (#109340)

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 JsonFile and JsonFileType classes
* 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.
* Split long RunWorkers._runtest() function into sub-functions.