Fix test failures on OS X

The reason for the test failures were because the regressions were doing a deterministic optimisation before sampling.  While the initial samples were close (still not sure why there weren't exact up to roundoff), the error propagated throughout the chain due to the random nature of the accept-reject procedure producing completely different chains.

A better approach is not to use every algorithm in one test, but test for regressions on samples using a vanilla sampler, and testing the bells and whistles when an analytical solution is available.  For example, the deterministic optimisation is tested when the MAP point is known.