patch 8.1.2041: no test for diff mode with syntax highlighting

Problem:    No test for diff mode with syntax highlighting.
Solution:   Add a test case.
diff --git a/src/version.c b/src/version.c
index bf0d350..370de36 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2041,
+/**/
     2040,
 /**/
     2039,