Added test cases for issue with embind and ASM_JS

This is to illustrate [Issue 1421](https://github.com/kripken/emscripten/issues/1421). It adds two test cases that run in the browser. Both make use of `embind` but only one of them uses `ASM_JS=1`. One of the two test cases fails, but it should pass.

    ./runner.py browser.test_embind_0 # Using -O0, passes
    ./runner.py browser.test_embind_2 # Using -O2, fails