patch 8.1.1581: shared functions for testing are disorganised

Problem:    Shared functions for testing are disorganised.
Solution:   Group finctions in script files. (Ozaki Kiichi, closes #4573)
diff --git a/src/version.c b/src/version.c
index e7ba35c..042c9c1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1581,
+/**/
     1580,
 /**/
     1579,