Added comment into runtests.py to explain why we're using DatabaseCache

runtests.py

@@ -82,6 +82,10 @@ if not settings.configured:
             'wagtail.wagtailredirects',
             'wagtail.tests',
         ],
+
+        # Using DatabaseCache to make sure that the cache is cleared between tests.
+        # This prevents false-positives in some wagtail core tests where we are
+        # changing the 'wagtail_root_paths' key which may cause future tests to fail.
         CACHES = {
             'default': {
                 'BACKEND': 'django.core.cache.backends.db.DatabaseCache',