Adding MathML test 024 to epub30-test-0100

Hi Markus,

This is a first pull request to test the waters. I'm adding a test for MathML horizontal stretch, including reference image rendering (I wasn't sure how to include the image, so let me know if you prefer it differently). 

I have four more tests to add for `epub30-test-100` and ~100 tests for a separate file.

Peter.