Set $wgInternalTidy to false in unit tests when running under HHVM

The Tidy extension for HHVM is nominally usable, but does not provide error
text returns. Tests should use TidySupport to determine whether they should use
the Tidy extension or not.

Change-Id: I0f842ddf8484206fc2a8b01289bc0a6750f34fed
Revert "Simplify MWTidy"

This is broken, for reasons indicated in
<https://gerrit.wikimedia.org/r/#/c/180384/>. It was broken before, but I made
it more broken. So revert for now, and I'll give this another stab.

Change-Id: I7e67a61f7d6370f90487be6470bebe1449432a4c