Test suite improvements.

* Deleted some mml,omml,tex files that had no counterparts in src/.
  (mrootE2, mtable1, noChildPresentation).  I assume these were here
  inadertently.  They were empty.

* Writer tests now start from .native files.  Previously we started
  from .tex and .mml files, which muddies things because the reader
  gets involved.

* Read files strictly in test suite to avoid "too many open files."