Remove test for <math> which fails when Extension:Math is installed

Remove test for <math> which fails when Extension:Math is installed

And add a test for <mw:toc>

C. Scott asked for this test to be added in CR on
Ib48b91238c3eddd6a86b62, but it was never actually executed prior to
Ic840678b851d6fc472b834 due to a TestFileReader bug. The nuisance it
causes now becomes apparent.

As was noted at the time, it is unnecessary because Tidy is not a
sanitization pass, <math> is not whitelisted in the Sanitizer. C. Scott
said "I was just concerned that the particular means used to hide the
mathml from tidy might also hide it from the sanitizer." I don't think
that's plausible, but we can test for it anyway by trying to pass
<mw:toc> elements through the Sanitizer.

Change-Id: I5928e6f31d8b80ef384a1c0e6bacfa789cac9770