Add YAML->RDF->YAML roundtripping to the tests

[mih]

It will become a key feature for us in the development of the SHACL-VUE data submission service. Better be tested properly.

[mih]

I am fighting with a bug that only revealed itself on a roundtrip. But this type of testing is still not done.

When playing, I noticed that a TTL->TTL conversion (no intermediate) via linkml-convert would have also revealed the issue. Maybe that is something cheap that could be done

[mih]

I tried this again, after 258040e

A simple TTL->TTL roundtrip isn't necessarily working for all examples, because sometimes multiple objects of one type are present in a single example. The conversion code doesn't know how to nest them.