Fix random test failures in testcase perfschema.mdl_func

The test case can get extra rows in its output from table
performance_schema.table_handles, left there by an earlier test case (for
example main.long_unique_delayed). So force a server restart at the
beginning of the test.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

mysql-test/suite/perfschema/r/mdl_func.result

+# restart
 UPDATE performance_schema.setup_instruments SET enabled = 'NO', timed = 'YES';
 UPDATE performance_schema.setup_instruments SET enabled = 'YES'
 WHERE name in ('wait/io/table/sql/handler',

mysql-test/suite/perfschema/t/mdl_func.test

 --source include/not_embedded.inc
 --source include/have_perfschema.inc
+# This test needs a fresh restart. The table performance_schema.table_handles
+# can otherwise contain extra rows left from previous testcases.
+# For example the test case main.long_unique_delayed, which uses
+# INSERT DELAYED, will leave extra rows in this table if run just before this
+# test, causing .result diff failure.
+--source include/restart_mysqld.inc
 
 UPDATE performance_schema.setup_instruments SET enabled = 'NO', timed = 'YES';