Update the testcase for MDEV-10330

Instead of restart_mysqld.inc, truncate the P_S tables that are used
by the test. We don't want to query some irrelevant data.

mysql-test/r/show_explain_ps.result

+truncate table performance_schema.events_statements_history_long;
+truncate table performance_schema.events_stages_history_long;
 drop table if exists t0, t1;
 select * from performance_schema.setup_instruments where name like '%show_explain%';
 NAME	ENABLED	TIMED

mysql-test/t/show_explain_ps.test

@@ -6,7 +6,10 @@
 # Like all other perfschema tests, we don't work on embedded server:
 --source include/not_embedded.inc
 
---source include/restart_mysqld.inc
+# There is a query below that selects from P_S tables.
+# Remove possible history that could confuse it
+truncate table performance_schema.events_statements_history_long;
+truncate table performance_schema.events_stages_history_long;
 
 --disable_warnings
 drop table if exists t0, t1;