[1.8.x] Prevented some test commands from needlessly running system checks.

This is a performance optimization and also fixes test errors with the
upcoming merge of contrib tests into tests/. The tests failed on MySQL
because the models with GeometryField were being checked but the
non-GIS MySQL backend didn't know how to handle them.

Backport of 197dd4b8f1 from master

tests/user_commands/tests.py

@@ -5,7 +5,7 @@ from django.core import management
 from django.core.management import BaseCommand, CommandError, find_commands
 from django.core.management.utils import find_command, popen_wrapper
 from django.db import connection
-from django.test import SimpleTestCase, ignore_warnings
+from django.test import SimpleTestCase, ignore_warnings, override_settings
 from django.test.utils import captured_stderr, captured_stdout, extend_sys_path
 from django.utils import translation
 from django.utils._os import upath
@@ -13,6 +13,14 @@ from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.six import StringIO
 
 
+# A minimal set of apps to avoid system checks running on all apps.
+@override_settings(
+    INSTALLED_APPS=[
+        'django.contrib.auth',
+        'django.contrib.contenttypes',
+        'user_commands',
+    ],
+)
 class CommandTests(SimpleTestCase):
     def test_command(self):
         out = StringIO()