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.
diff --git a/src/version.c b/src/version.c
index d4c872a..926efea 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    456,
+/**/
     455,
 /**/
     454,