[2.1.x] Fixed MySQL QuerySet.explain() test when running tests in reverse.

Backport of 13fe5a87f9554a6371f1d0914887bffcdf978a8a from master

tests/queries/test_explain.py

@@ -69,6 +69,9 @@ class ExplainTests(TestCase):
 
     @unittest.skipUnless(connection.vendor == 'mysql', 'MySQL specific')
     def test_mysql_text_to_traditional(self):
+        # Initialize the cached property, if needed, to prevent a query for
+        # the MySQL version during the QuerySet evaluation.
+        connection.features.needs_explain_extended
         with CaptureQueriesContext(connection) as captured_queries:
             Tag.objects.filter(name='test').explain(format='text')
         self.assertEqual(len(captured_queries), 1)