[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/admin_scripts/tests.py

@@ -1589,6 +1589,7 @@ class CommandTypes(AdminScriptTestCase):
         being executed (#21255).
         """
         command = BaseCommand(stderr=StringIO())
+        command.check = lambda: []
         command.handle = lambda *args, **kwargs: args
         with mock.patch('django.core.management.base.connections') as mock_connections:
             command.run_from_argv(['', ''])