Run simulator equivalence tests in double precision only

Run simulator equivalence tests in double precision only

The simulator equivalence tests are a great help in the effort of
rewriting the simulator framework, but might be too sensitive for
general use. They have been shown to be extremely sensitive to
order of operations which inevitably changes in day-by-day
development. Most recent test failures could be traced back to
the absence of a parenthesis in otherwise identical code (for
md-vv), and to the differences between the SIMD and non-SIMD
code paths (for md), respectively. Double precision builds seem to
be less affected by such re-ordering, so this commit changes the
simulator equivalence tests to be only enabled for double precision
builds. As the simulator rewriting progresses, and the new code paths
get covered by the existing tests, we might want to move away from
these type of equivalence tests altogether.

Change-Id: Ia4ad269959efd207242795ecedb88eaf7a870589