MDEV-10330: main.show_explain_ps fails sporadically in buildbot

The reason is that selecting from events_waits_history_long creates
a race condition: it can happen that SHOW EXPLAIN thread was kicked
off CPU exactly after posting a SHOW EXPLAIN request and then it wont
need to wait.

It doesn't seem to make sense to add more waits to stabilize the testcase.
Let's instead make a check that SHOW EXPLAIN statement has a
"stage/sql/show explain" stage.

mysql-test/r/show_explain_ps.result

@@ -28,8 +28,13 @@ count(*)
 10
 set debug_dbug='';
 select event_name
-from performance_schema.events_waits_history_long 
-where event_name='wait/synch/cond/sql/show_explain';
+from
+performance_schema.events_stages_history_long
+where
+event_name like '%show explain' and
+thread_id in(select thread_id
+from performance_schema.events_statements_history_long
+where EVENT_NAME='statement/sql/show_explain');
 event_name
-wait/synch/cond/sql/show_explain
+stage/sql/show explain
 drop table t0;

mysql-test/t/show_explain_ps.test

@@ -6,6 +6,8 @@
 # Like all other perfschema tests, we don't work on embedded server:
 --source include/not_embedded.inc
 
+--source include/restart_mysqld.inc
+
 --disable_warnings
 drop table if exists t0, t1;
 --enable_warnings
@@ -45,7 +47,11 @@ reap;
 set debug_dbug='';
 
 select event_name
-from performance_schema.events_waits_history_long 
-where event_name='wait/synch/cond/sql/show_explain';
-
+from
+performance_schema.events_stages_history_long
+where
+  event_name like '%show explain' and
+  thread_id in(select thread_id
+               from performance_schema.events_statements_history_long
+               where EVENT_NAME='statement/sql/show_explain');
 drop table t0;