Improve CI testing of portdb script

[erikjohnston]

Edit: original title was "Add check to CI that sequences are correctly ported by port script"

c.f. #8387

[erikjohnston]

It would be good to fix a more general thing which is that we don't currently check that the ported DB is usable. Some things that we can do are:

1. Ensure that a server can successfully start up using the ported DB
2. Ensure that we can register/login/create room/send event, which should check the most common code paths
3. Run the unit tests or SyTest using the ported DB as a base

[DMRobertson]

#13226 fixed a particularly gnarly bug where portdb was capable of dropping rows with negative row IDs.

A check e.g. that SELECT count(*) FROM <tablename> queries were consistent between sqlite and postgres would have spotted the problem.