patch 8.1.1251: no test for completion of mapping keys

Problem:    No test for completion of mapping keys.
Solution:   Add a test.  Also clean up the code.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1251,
 /**/
     1250,
 /**/