Add RoleType Concept tests

[alexjpwalker]

Problem to Solve

RoleType Concept methods were all broken in client-java and I didn't know about it, because we have no RoleType Concept tests.

Proposed Solution

Add RoleType Concept tests

[haikalpribadi]

Please make sure that these new scenarios pass in Grakn Core 2.0 too before merging, @alexjpwalker.

[alexjpwalker]

We still don't have this, but most RoleType methods are in fact tested anyway - either directly in a RelationType test, or indirectly by being called from other methods in RelationType, which are themselves tested. So I don't think this is urgent, and may in fact not be needed at all.