MDEV-19803 Long semaphore wait error on galera.MW-388

The long semaphore wait appeared to be the caused by the following
pattern in the MTR test:

```
SET DEBUG_SYNC = "now SIGNAL wsrep_after_certification_continue";
SET DEBUG_SYNC = "now SIGNAL signal.wsrep_apply_cb;
```

Raising two signals, one right after another, caused one signal to
overwrite the other, before the signal was consumed by the thread.
This caused one thread to be stuck until the debug sync point would
timeout.

mysql-test/suite/galera/disabled.def

@@ -12,7 +12,6 @@
 
 MW-286 : MDEV-18464 Killing thread can cause mutex deadlock if done concurrently with Galera/replication victim kill
 MW-329 : MDEV-19962 Galera test failure on MW-329
-MW-388: MDEV-19803 Long semaphore wait error on galera.MW-388	
 galera_account_management : MariaDB 10.0 does not support ALTER USER
 galera_as_master_gtid : Requires MySQL GTID
 galera_as_master_gtid_change_master : Requires MySQL GTID

mysql-test/suite/galera/r/MW-388.result

@@ -18,12 +18,10 @@ connection node_1a;
 SET SESSION DEBUG_SYNC = "now WAIT_FOR sync.wsrep_apply_cb_reached";
 connection node_1;
 SET SESSION wsrep_sync_wait = 0;
-SET SESSION DEBUG_SYNC = 'wsrep_after_replication SIGNAL wsrep_after_replication_reached WAIT_FOR wsrep_after_replication_continue';
 CALL insert_proc ();;
 connection node_1a;
-SET SESSION DEBUG_SYNC = "now WAIT_FOR wsrep_after_replication_reached";
+SET SESSION wsrep_sync_wait = 0;
 SET GLOBAL debug_dbug = "";
-SET DEBUG_SYNC = "now SIGNAL wsrep_after_replication_continue";
 SET DEBUG_SYNC = "now SIGNAL signal.wsrep_apply_cb";
 connection node_2;
 connection node_1;

mysql-test/suite/galera/r/galera_ist_restart_joiner.result

@@ -13,7 +13,6 @@ Loading wsrep_provider ...
 SET SESSION wsrep_on=OFF;
 SET SESSION wsrep_on=ON;
 connection node_1;
-connection node_1;
 UPDATE t1 SET f2 = 'd' WHERE f1 > 3;
 connection node_2;
 connection node_1;

mysql-test/suite/galera/t/MW-388.test

@@ -31,6 +31,8 @@ DELIMITER ;|
 SET GLOBAL wsrep_slave_threads = 2;
 SET GLOBAL debug_dbug = "d,sync.wsrep_apply_cb";
 
+--let $expected_cert_failures = `SELECT VARIABLE_VALUE + 1 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_local_cert_failures'`
+
 --connection node_2
 --send INSERT INTO t1 VALUES (1, 'node 2');
 
@@ -40,14 +42,14 @@ SET SESSION DEBUG_SYNC = "now WAIT_FOR sync.wsrep_apply_cb_reached";
 
 --connection node_1
 SET SESSION wsrep_sync_wait = 0;
-SET SESSION DEBUG_SYNC = 'wsrep_after_replication SIGNAL wsrep_after_replication_reached WAIT_FOR wsrep_after_replication_continue';
 --send CALL insert_proc ();
 
 --connection node_1a
-SET SESSION DEBUG_SYNC = "now WAIT_FOR wsrep_after_replication_reached";
+SET SESSION wsrep_sync_wait = 0;
+--let $wait_condition = SELECT VARIABLE_VALUE = $expected_cert_failures FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_local_cert_failures'
+--source include/wait_condition.inc
 
 SET GLOBAL debug_dbug = "";
-SET DEBUG_SYNC = "now SIGNAL wsrep_after_replication_continue";
 SET DEBUG_SYNC = "now SIGNAL signal.wsrep_apply_cb";
 
 --connection node_2