MDEV-20157 perfschema.stage_mdl_function failed in buildbot with wrong result

MDL wait consists of short 1 second waits (this is not configurable)
repeated until lock_wait_timeout is reached. The stage is changed
to Waiting and back every second. To have predictable result in the
test the query should filter all sequences of X, "Waiting for MDL", X,
leaving just X.

mysql-test/suite/perfschema/include/stage_setup.inc

@@ -121,12 +121,23 @@ begin
       where thread_id = my_thread_id
       and nesting_event_id = my_statement_id
       order by event_id asc;
-    # Ignore query cache as it may not be enabled
-    select username, event_name, nesting_event_type
-      from performance_schema.events_stages_history
-      where thread_id = my_thread_id
-      and nesting_event_id = my_statement_id and
-      event_name not like "%query cache%"
+    # 1. Ignore query cache as it may not be enabled
+    # 2. MDL wait consists of short 1 second waits (this is not configurable)
+    #    repeated until lock_wait_timeout is reached. The stage is changed
+    #    to Waiting and back every second. To have predictable result here
+    #    the query filters all sequences of X, "Waiting for MDL", X,
+    #    leaving just X.
+    with h as (
+      select event_id, username, event_name, nesting_event_type,
+        lag(event_name) over (order by event_id) = lead(event_name) over (order by event_id)
+        and event_name = "stage/sql/Waiting for stored function metadata lock" as v1,
+        lag(event_name, 2) over (order by event_id) = event_name
+        and lag(event_name, 1) over (order by event_id) = "stage/sql/Waiting for stored function metadata lock" as v2
+        from performance_schema.events_stages_history
+        where thread_id = my_thread_id
+        and nesting_event_id = my_statement_id and
+        event_name not like "%query cache%"
+      ) select username, event_name, nesting_event_type from h where v1 is not true and v2 is not true
       order by event_id asc;
   end;
   else