MDEV-7205 : Galera cluster & sql_log_bin = off don't work

While sql_bin_log=1(0) is meant to control binary logging for the
current session so that the updates to do(not) get logged into the
binary log to be replicated to the async MariaDB slave. The same
should not affect galera replication.

That is, the updates should always get replicated to other galera
nodes regardless of sql_bin_log's value.

Fixed by making sure that the updates are written to binlog cache
irrespective of sql_bin_log.

Added test cases.

sql/sql_class.cc

@@ -1440,6 +1440,8 @@ void THD::init(void)
   else
     variables.option_bits&= ~OPTION_BIN_LOG;
 
+  variables.sql_log_bin_off= 0;
+
   select_commands= update_commands= other_commands= 0;
   /* Set to handle counting of aborted connections */
   userstat_running= opt_userstat_running;