MDEV-7342: Test failure in perfschema.setup_instruments_defaults

Fix a possible race in the test case when restarting the server.

Make sure we have disconnected before waiting for the reconnect
that signals that the server is back up. Otherwise, we may in
rare cases continue the test while the old server is shutting
down, eventually leading to "connection lost" failure.

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

@@ -60,6 +60,8 @@ WHERE name like "%wait/io/table/sql/handler%";
 --echo #
 --echo # Stop server
 --send_shutdown
+--source include/wait_until_disconnected.inc
+
 --echo # Restart server with wait/io/table/sql/handler disabled
 
 --exec echo "restart:--loose-performance-schema-instrument=%wait/io/table/sql/%=off" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect