patch 7.4.1838

Problem:    Functions specifically for testing do not sort together.
Solution:   Rename garbagecollect_for_testing() to test_garbagecollect_now().
            Add test_null_list(), test_null_dict(), etc.

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1838,
 /**/
     1837,
 /**/