[2.2.x] Fixed #30111 -- Fixed AppRegistryNotReady error with django.contrib.postgres in INSTALLED_APPS.

Regression in e192223ed9.
Backport of 2804b8d215 from master.

tests/postgres_tests/integration_settings.py

@@ -0,0 +1,5 @@
+SECRET_KEY = 'abcdefg'
+
+INSTALLED_APPS = [
+    'django.contrib.postgres',
+]

tests/postgres_tests/test_integration.py

@@ -0,0 +1,17 @@
+import os
+import subprocess
+import sys
+
+from tests.postgres_tests import PostgreSQLSimpleTestCase
+
+
+class PostgresIntegrationTests(PostgreSQLSimpleTestCase):
+    def test_check(self):
+        os.chdir(os.path.dirname(__file__))
+        result = subprocess.run(
+            [sys.executable, '-m', 'django', 'check', '--settings', 'integration_settings'],
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.PIPE,
+        )
+        stderr = '\n'.join([e.decode() for e in result.stderr.splitlines()])
+        self.assertEqual(result.returncode, 0, msg=stderr)