docs(core): say which database the tests actually need

The helper told you to use "an isolated database for tests", which reads
as a database of its own and is not what the suite wants. Route tests
reach the database through the app and core tests reach it directly, so
two different values put the fixtures in one database and the assertions
in the other — around forty failures, none of them in the code that
caused it, and nothing in the output naming the setting.

Also drops a type import nothing uses.

packages/auth/src/issuer.ts

@@ -220,7 +220,6 @@ import {
 	StorageRefreshStore
 } from './refresh.js';
 import {
-	type DeviceGrant,
 	type DeviceGrantSubject,
 	type DeviceStore,
 	hashDeviceCode,