Example test cases for common edge cases in state resolution

Nobody has claimed this yet.

As a positive example, the section for Canonical JSON has an [examples subsection](https://spec.matrix.org/v1.12/appendices/#examples) which shows how to encode a few JSON objects into Canonical JSON.

I think a similar set of prescribed example test cases for state resolution would be very helpful for validating an implementation. I imagine each example would have a set of PDUs as input, and the output could be:

* the resolved state
* a list of edges in the auth chain graph of the resolved state
* this would help ensure that the algorithm reaches the correct state by the correct path, rather than doing something wrong but still ending up with the right answer somehow
* the set of rejected events, and ideally the reasons why these events were rejected
* maybe other stuff I'm not thinking of

I acknowledge that because the data involved here would be rather large, it might be hard to actually fit this into the spec somewhere, and I don't have any suggestions for overcoming that.

Keywords: test vectors

Start by reviewing the linked Canonical JSON examples subsection and the state resolution specification. Define a format and location for prescribed PDU inputs, resolved state, auth-chain edges, rejected events, and rejection reasons. Done means the proposed edge-case vectors are agreed and documented sufficiently to validate independent implementations.

Written by the indexing model from the issue text.