Fix covmat reg tests
Created by: wilsonmr
I actually kept the old test and added another because I think that the procedure before wasn't robust and the test actually helped find this issues. To (hopefully) fix that I added in an abs() around the correlation matrix eigenvalue in case it is close to zero but negative
Then I added another test which checks with a slightly more sane matrix. Also using numpy.testing.assert_allclose whereever possible in that test module for better errors
closes #777 (closed)