tests: add tests for statement_timeout

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

test/io/db_config.sql

@@ -59,10 +59,15 @@ ALTER ROLE other_authenticator SET pgrst.db_extra_search_path = 'public, extensi
 ALTER ROLE other_authenticator SET pgrst.openapi_mode = 'disabled';
 ALTER ROLE other_authenticator SET pgrst.openapi_security_active = 'false';
 
--- limited authenticator used for failed schema cache loads
-CREATE ROLE limited_authenticator LOGIN NOINHERIT;
+-- authenticator used for tests that manipulate statement timeout
+CREATE ROLE timeout_authenticator LOGIN NOINHERIT;
 
-create or replace function no_schema_cache_for_limited_authenticator() returns void as $_$
+create function set_statement_timeout(role text, milliseconds int) returns void as $_$
 begin
-  ALTER ROLE limited_authenticator SET statement_timeout to 1;
-end $_$ volatile security definer language plpgsql ;
+  execute format($$
+    alter role %I set statement_timeout to %L;
+  $$, role, milliseconds);
+end $_$ volatile security definer language plpgsql;
+
+-- authenticator used for test-independent database manipulation
+CREATE ROLE meta_authenticator LOGIN NOINHERIT;