The new tests verify that:
- statement_timeout on the authenticator role works to cancel slow statements
- changes to statement_timeout take effect on SIGUSR1
This reuses the old "limited_authenticator" role and adds some plumbing to
allow reliably changing the statement timeout even if the current role is
not functional due to a low statement timeout, and to make tests that modify
the role independent from each other.
- introduce module-wide metapostgrest fixture to have an out-of-band way to
manipulate the database, without having to spin up extra postgrest instances
per test
- reset statement_timeout at the start of the respective tests
