Implement variable roles Match tests

[flyingsilverfin]

A lot of nuanced match cases can be found by considering variable roles in a relation. A quick check shows that we have no such match tests in the current match.feature.

For example, see the test added in #171.

We should write tests cases to cover these, and discuss what the expected outcome of for instance the following queries are:

match ($role: $x) isa $rel; $role type friendship:friend;

vs

match ($role: $x) isa rel; $role sub friendship:friend;

and any other fun cases that can be found!