MDEV-9975 - main.partition_innodb_plugin fails sporadically

Fixed wait condition to wait for InnoDB to actually acquire row locks instead
of waiting for thread to enter "Searching rows for update" state (which happens
earlier).

Also use default wait timeout, 2 seconds can be too few for threads to enter
appropriate state.

mysql-test/t/partition_innodb_plugin.test

@@ -124,12 +124,8 @@ SEND;
 UPDATE `t``\""e` SET a = 12 WHERE a = 0;
 --echo # default connection
 connection default;
-let $wait_timeout= 2;
-let $wait_condition= SELECT 1 FROM INFORMATION_SCHEMA.PROCESSLIST
-WHERE ID = $id_1 AND STATE = 'Searching rows for update';
+let $wait_condition= SELECT COUNT(*)=2 FROM INFORMATION_SCHEMA.INNODB_LOCKS;
 --source include/wait_condition.inc
-#--echo # tested wait condition $wait_condition_reps times
-# INNODB_LOCKS only exists in innodb_plugin
 --sorted_result
 SELECT lock_table, COUNT(*) FROM INFORMATION_SCHEMA.INNODB_LOCKS
 GROUP BY lock_table;