patch 8.0.1721: no test for using the 'termsize' option

Problem:    No test for using the 'termsize' option.
Solution:   Add a test.
diff --git a/src/version.c b/src/version.c
index 2d28206..851f050 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1721,
+/**/
     1720,
 /**/
     1719,