Added OMML reader tests.

Note:  I just generated the native files using the current
texmath.  THe output isn't perfect, but at least this gives us
a way to see what changes after we make revisions.

I waited til the round-trip tests were pretty confidence-inspiring
before doing this.

I also looked at the results in Word, and things look OK on
inspection.

Feel free to change, add, or subtract tests.  Note that these
tests all start with OMML generated by texmath.  IT would be good
to have tests on Word-generated OMML.