Fix & update docs for GMX_TEST_OPTIONS

The macro did not appear in Doxygen documentation, because it was
excluded to avoid warnings from undocumented instances of it.  Since
there aren't that many, opted to protect those with \cond instead.  Also
some minor updates and additions to the documentation, and some minor
cleanup of touched code.

Change-Id: I04785e6d66f124ae8ec5dde87bb7c7442230d83a