Fix usage of w3id.org/bench, w3id.org/system

[denkv]

In tests.

[MichaelRoeder]

@denkv can you please extend the explanation for this? What exactly is the target? Should only URIs of w3id be used or do some of the URIs have issues?

[denkv]

We do not have these prefixes on w3id.org allocated to us, so we should just use something different.