test: refactor: Better organize validatePostResponse tests
test: refactor: Better organize validatePostResponse tests

Before, these tests were spread out in 3 sub-sections. One was
not labeled and the other two were not labeled as being for
validatePostResponse.

Now, more tests for this function are grouped together and clearly
labeled, more consistent with how other tests in the file are handled.

cjbarth/passport-saml,bergie/passport-saml,cjbarth/passport-saml