Add tests for propertyNames with const/enum

[michaelmior]

This checks for the correct functioning of propertyNames with a const or enum schema.
This inspired by a bug reported in the Blaze validator for this specific case.

[karenetheridge]

I'm curious what this implementation did to cause those particular keywords to not work properly?

[jviotti]

@karenetheridge It was totally an oversight. The code for enum/const would not take into consideration the fact that the target could be a key and not the instance, like in other keywords.