Type `tests.utils` (#13028)

* Cast to postgres types when handling postgres db

* Remove unused method

* Easy annotations

* Annotate create_room

* Use `ParamSpec` to annotate looping_call

* Annotate `default_config`

* Track `now` as a float

`time_ms` returns an int like the proper Synapse `Clock`

* Introduce a `Timer` dataclass

* Introduce a Looper type

* Suppress checking of a mock

* tests.utils is typed

* Changelog

* Whoops, import ParamSpec from typing_extensions

* ditch the psycopg2 casts