FYI: Rename of `tests` directory to just `test`

[Sam Clegg]

Hoping to land this change today which shouldn't effect any emscripten users but certainly will affect emscripten developers (at least those who write tests!): https://github.com/emscripten-core/emscripten/pull/17502.

Since git seems to be quite good at rebasing across renames I don't think this should be too annoying.  Existing changes which include tests should rebase without any trouble.  

cheers,

sam