gh-109276: libregrtest: WASM use stdout for JSON (#109355)

gh-109276: libregrtest: WASM use stdout for JSON (#109355)

On Emscripten and WASI, or if --python command line is used,
libregrtest now writes JSON into stdout, instead of using a name
file.

* Add JsonFileType.STDOUT.
* Remove JsonFileType.FILENAME.
* test.pythoninfo logs environment variables related to
  cross-compilation and running Python on Emscripten/WASI.