gh-84461: Drop -sWASM, fix building tests for browser (GH-91530)

- drop unnecessary ``=1`` suffix from Emscripten flags
- drop unnecessary ``-sWASM`` flag for side modules
- rename ``build_platform`` to ``build_wasm``. I introduced the target
  for WASM builds a couple of months ago.
- fix ``--enable-test-modules`` for browser builds