Add `assertSameDictionaryKeysValues` to compare keys and values without regard to key order

[samlitowitz]

Resolves #6088

@sebastianbergmann assuming you are ok with the concept, I would love your input on the implementation. Notably a large portion of the constraint work is take from the ArrayComparator and the ObjectComparator making me wonder if a new comparator was the right approach. If it's ok with you, I wanted to have the discussion here before opening issues or making PR's against the comparator repository.