patch 8.0.0538: no test for falling back to default term value

Problem:    No test for falling back to default term value.
Solution:   Add a test.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    538,
 /**/
     537,
 /**/