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.
diff --git a/src/version.c b/src/version.c
index 878891e..e18c025 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    531,
+/**/
     530,
 /**/
     529,