Test TemplateLookup dirs are not cleared during lms startup.

LMS-2498

lms/tests.py

+"""Tests for the lms module itself."""
+
+from django.test import TestCase
+
+from edxmako import add_lookup, LOOKUP
+from lms import startup
+
+class TemplateLookupTests(TestCase):
+    """
+    Tests for TemplateLookup.
+    """
+
+    def test_add_lookup_to_main(self):
+        """Test that any template directories added are not cleared when microsites are enabled."""
+
+        add_lookup('main', 'external_module', __name__)
+        directories = LOOKUP['main'].directories
+        self.assertEqual(len([dir for dir in directories if 'external_module' in dir]), 1)
+
+        # This should not clear the directories list
+        startup.enable_microsites()
+        directories = LOOKUP['main'].directories
+        self.assertEqual(len([dir for dir in directories if 'external_module' in dir]), 1)