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.
diff --git a/src/version.c b/src/version.c
index dc7cfb5..8de7e66 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    538,
+/**/
     537,
 /**/
     536,