[PSA] Emscripten test suite now runs against a pinned emsdk version

[Sam Clegg]

This is a PSA for emscripten developers only.   You can stop reading now if you don't work on emscripten itself.

We now run the emscripten tests against a pinned version of emsdk.  The main advantage of this is that codesize tests can no longer become out-of-date on the main branch.

If you work on emscripten and want to run the codesize tests it is recommended you install the exact version of emsdk that emscripten currently expects.  This is stored in `test/emsdk_version.txt` and you can install it using e.g:

   emsdk install $(cat test/emsdk_version.txt)

This file will update fairly often (likely at least once per day) so it's recommended to run this command frequently.   You might even want to add it to a git post-checkout script.

cheers,

sam