BUG#25211: events_bugs.test fails on sapsrv1

The problem was that the events_bugs test could randomly fail due to
races in the test case.

The solution is to replace fixed sleeps with reliable polling of a
certain state to settle.  For that, a new auxiliary script
include/wait_condition.inc is used, that allows waiting for a given
query to return true.

mysql-test/include/wait_condition.inc

@@ -0,0 +1,39 @@
+# include/wait_condition.inc
+#
+# SUMMARY
+#
+#    Waits until the passed statement returns true, or the operation
+#    times out.
+#
+# USAGE
+#
+#    let $wait_condition=
+#      SELECT c = 3 FROM t;
+#    --source include/wait_condition.inc
+#
+# EXAMPLE
+#    events_bugs.test
+#
+
+--disable_query_log
+
+let $wait_counter= 300;
+while ($wait_counter)
+{
+    let $success= `$wait_condition`;
+    if ($success)
+    {
+        let $wait_counter= 0;
+    }
+    if (!$success)
+    {
+        real_sleep 0.1;
+        dec $wait_counter;
+    }
+}
+if (!$success)
+{
+  echo Timeout in wait_condition.inc for $wait_condition;
+}
+
+--enable_query_log