Test whether a phyloref resolves to multiple nodes on a single phylogeny

[gaurav]

Currently, we test whether a phyloref has resolved to every node that it is expected to resolve to. However, we don't test whether that expected resolution information makes sense. For instance:

• A phyloreference might not be expected to resolve to any node on any phylogeny, in which case we should report it as untestable.
• A phyloreference may be expected to resolve to multiple nodes on the same phylogeny. We should probably emit a warning about this.