Add test for `@graph` keyword aliasing

[lolgab]

I think I found an issue in titanium-json-ld which seems to not alias properly the @graph.

Since it uses the tests in this repository, I then noticed that there aren't any tests checking that an implementation can properly alias the @graph name.

From what I read in https://www.w3.org/TR/json-ld11/#keywords:

The @graph keyword MAY be aliased

So I assume that there should be a test to validate that the JSON-LD framing algorithm can handle the @graph alias.