tests with blank nodes

[elf-pavlik]

just fast

$ grep blank test/*
$ grep _: test/*

makes impression that we don't test cases with blank nodes 🃏

[mcollina]

That's fun :).

I'm giving you the commit bit in this one too, if it's ok for you.

[mcollina]

I think the generation of blank node using uuids of JSON-LD is right.
How about we bring that to its own package not depending on levelgraph?
Then it can be used kind of everywhere it's needed.