patch 8.1.0456: running test hangs when the input file is being edited

Problem:    Running test hangs when the input file is being edited.
Solution:   Use a SwapExists autocommand to ignore editing the test script.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    456,
 /**/
     455,
 /**/