patch 8.1.0520: screen diff test sometimes fails

Problem:    Screen diff test sometimes fails.
Solution:   Add to list of flaky tests.
diff --git a/src/version.c b/src/version.c
index ad815fb..e3491a0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    520,
+/**/
     519,
 /**/
     518,