Add Emscripten tests

[mcbarton]

Once we have wasm working in xeus-cpp it may be worth trying to make use of the Emscripten test suite https://emscripten.org/docs/getting_started/test-suite.html

[anutosh491]

Can this be closed now that we have testing for our emscripten build ?

cc @mcbarton

[anutosh491]

Closing as I think this has been addressed. Can reopen if any disucssion is pending !