patch 8.1.0531: flaky tests often fail with a common error message

Problem:    Flaky tests often fail with a common error message.
Solution:   Add a pattern to match an error message indicating a flaky test.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    531,
 /**/
     530,
 /**/