pvalues didn't use df_resid_inference which is used to separate it from df_resid in cluster robust standard errors.
adds one unit test for pvalues and conf_int
comment to unit tests
all sandwich unit tests should test the full results, which they don't do for the new fit options
tests needs streamlining. I'm not even sure how much each of the versions of choosing covariances and use_t are tested.
some asserts might be duplicated, see CheckOLSRobustNewMixin
BUG: LikelihoodModelResults.conf_int use df_resid_inference closes #1903
can be merged when TravisCI is green
Coverage increased (+0.0%) when pulling ea8777b on josef-pkt:bug_pvalues_use_t_1903 into 93efca1 on statsmodels:master.
I forgot to rewrite the commit message.
This fixes pvalues and not conf_int