MDEV-13549 Fix and re-enable test galera.MW-286

This test failed to work properly because the fixes it came
with were not merged from upstream.
The test would fail with a spurious ER_LOCK_DEADLOCK error
for a conflict that happened earlier in the test execution,
while wsrep is disabled.
The original fix was to set THD::wsrep_conflict_state only
if wsrep is enabled (see wsrep_thd_set_conflict_state() in
sql/wsrep_mysqld.cc)

mysql-test/suite/galera/disabled.def

@@ -48,7 +48,6 @@ galera_toi_ddl_nonconflicting : MDEV-13549 Galera test failures
 galera_parallel_simple : MDEV-13549 Galera test failures
 galera_admin : MDEV-13549 Galera test failures
 galera_var_max_ws_rows :  MDEV-13549 Galera test failures 10.1
-MW-286 : MDEV-13549 Galera test failures 10.1
 galera_as_master: MDEV-13549 Galera test failures 10.1
 galera_pc_ignore_sb : MDEV-13549 Galera test failures 10.1
 galera_lock_table : MDEV-13549 Galera test failures 10.1

mysql-test/suite/galera/t/MW-286.test

@@ -25,7 +25,6 @@ SET wsrep_on = FALSE;
 --error ER_QUERY_INTERRUPTED
 ALTER TABLE t1 ADD PRIMARY KEY (f1);
 
-SET SESSION wsrep_sync_wait = 0;
 SET wsrep_on = TRUE;
 SET GLOBAL wsrep_desync = FALSE;
 

sql/wsrep_mysqld.cc

@@ -2336,7 +2336,7 @@ extern "C" void wsrep_thd_set_query_state(
 
 void wsrep_thd_set_conflict_state(THD *thd, enum wsrep_conflict_state state)
 {
-  thd->wsrep_conflict_state= state;
+  if (WSREP(thd)) thd->wsrep_conflict_state= state;
 }