BUG: LikelihoodModelResults.pvalues use df_resid_inference

closes #1903

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