patch 8.1.0596: not all parts of printf() are tested

Problem:    Not all parts of printf() are tested.
Solution:   Add a few more test cases. (Dominique Pelle, closes #3691)
diff --git a/src/version.c b/src/version.c
index 83026ba..577be31 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    596,
+/**/
     595,
 /**/
     594,