Stopped special-casing postgres-specific tests

Refs #23879.

tests/postgres_tests/__init__.py

@@ -0,0 +1,9 @@
+import unittest
+
+from django.db import connection
+from django.test import TestCase
+
+
+@unittest.skipUnless(connection.vendor == 'postgresql', "PostgreSQL specific tests")
+class PostgresSQLTestCase(TestCase):
+    pass