ipc_*: Formal test coverage analysis. #84

Filed by @ygoldfeld pre-open-source:

We should formally analyze coverage of the source code. Consider doing so both before and after #83.

• Before: So we know how to do it and have a good idea of how much we actually lack (the current text in ipc_{core|transport_structured|session|shm}: Unit tests. #83 description is my subjective assessment).
• After: So we know what might still be lacking; and possibly to set up automatic periodic coverage analysis (perhaps for each PR).