Add a very simple Emscripten-specific test.

This will be expanded on later to be a lot more complete.

tests/_build/README.md

@@ -6,3 +6,18 @@ Build and run from this directory. Example:
     ./bin/test_deviceio
     
 Output files will be placed in the "res/output" folder.
+
+
+Emscripten
+----------
+On Windows, you need to move into the build and run emsdk_env.bat from a command prompt using an absolute
+path like "C:\emsdk\emsdk_env.bat". Note that PowerShell doesn't work for me for some reason. Examples:
+
+    emcc ../test_emscripten/ma_test_emscripten.c -o bin/test_emscripten.html-sAUDIO_WORKLET=1 -sWASM_WORKERS=1 -sASYNCIFY -DMA_ENABLE_AUDIO_WORKLETS -Wall -Wextra
+    
+If you output WASM it may not work when running the web page locally. To test you can run with something
+like this:
+
+    emrun ./bin/test_emscripten.html
+
+If you want to see stdout on the command line when running from emrun, add `--emrun` to your emcc command.