Add multi-graph tests for weighted similarity algorithms

[ChuckHastings]

Weighted similarity tests are currently disabled for RMAT. This is due to the fact that RMAT generates a multigraph, and the reference implementation for weighted similarity and the cugraph primitive-based implementation for weighted similarity generate different results for a multi-graph.

This needs to be investigated. I suspect the issue is that the desired behavior is not clear in the presence of a multi-graph. So we probably first need to decide what the right behavior is in the case of a multi-graph. Then we need to update the reference implementation to reflect the proper behavior. We might also need to implement the primitive-based implementation - unless the proper behavior is what's currently implemented by the primitive.