TST: enable tests for llf after change to WLS.loglike see #1170

This enables tests that were commented out.

It is supposed to go in after fixing WLS.loglike as discussed in #1170

I cannot find the commit for WLS.loglike in any of bashtage's PRs right now.
If the change is not included in any of bashtage's PRs, I will make a separate PR, that also fixes the failing tests because we will use a different definition or normalization than Stata.


the tests don't fail with current master:

I had changed GLSAR to subclass GLS instead of WLS which has the correct loglike
GLSHet which subclasses WLS doesn't have unit tests yet. (I didn't find a direct equivalent in Stata or R, IIRC)