Refs #31117 -- Made various tests properly handle unexpected databases aliases.

- Used selected "databases" instead of django.db.connections.
- Made routers in tests.migrations skip migrations on unexpected
  databases.
- Added DiscoverRunnerGetDatabasesTests.assertSkippedDatabases() hook
  which properly asserts messages about skipped databases.

tests/auth_tests/test_admin_multidb.py

@@ -3,7 +3,6 @@ from unittest import mock
 from django.contrib import admin
 from django.contrib.auth.admin import UserAdmin
 from django.contrib.auth.models import User
-from django.db import connections
 from django.test import TestCase, override_settings
 from django.urls import path, reverse
 
@@ -32,7 +31,7 @@ class MultiDatabaseTests(TestCase):
     @classmethod
     def setUpTestData(cls):
         cls.superusers = {}
-        for db in connections:
+        for db in cls.databases:
             Router.target_db = db
             cls.superusers[db] = User.objects.create_superuser(
                 username='admin', password='something', email='test@test.org',
@@ -40,7 +39,7 @@ class MultiDatabaseTests(TestCase):
 
     @mock.patch('django.contrib.auth.admin.transaction')
     def test_add_view(self, mock):
-        for db in connections:
+        for db in self.databases:
             with self.subTest(db_connection=db):
                 Router.target_db = db
                 self.client.force_login(self.superusers[db])