Add tests for `$dynamicRef` keyword that contains JSON Pointer

[flaksp]

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Currently, all tests cover only the case where $dynamicRef contains a plain name fragment, e.g.:

{
  "$dynamicRef": "#items"
}

I believe we need more test cases because the question "what will happen if $dynamicRef will contain something but plain name fragment" seems to be popular from people like me who try to investigate how dynamic scope works. And it's confusing that there are no official examples nor in the spec and in the tests.