Test for isolated subgraphs

[langston-barrett]

We might want to define and test for some high level CPG invariants, such as:

• Do we expect our CPG to be connected, i.e. for there to be a set of edges from any node to any other node?
• If not, do we expect certain nodes or communities of nodes to always be orphaned/unconnected?

Migrated from internal (Gitlab) MATE issue number(s) 624