MXS-3249 Fix mysqltest tests

To avouid failures caused by default MariaDB configuration files
containing entries incompatible with mysqltest, mysqltest is
invoked with the glah --no-defaults, which will cause it *not* to
read any configuration files.

system-test/masking_user.sh

@@ -34,7 +34,8 @@ dir="$src_dir/masking/$1"
 
 user=skysql
 test_name=masking_user
-mysqltest --host=${maxscale_000_network} --port=$port \
+mysqltest --no-defaults \
+          --host=${maxscale_000_network} --port=$port \
           --user=$maxscale_user --password=$maxscale_password \
           --logdir=$logdir \
           --test-file=$dir/t/$test_name.test \
@@ -50,7 +51,8 @@ fi
 
 user=maxskysql
 test_name=masking_user
-mysqltest --host=${maxscale_000_network} --port=$port \
+mysqltest --no-defaults \
+          --host=${maxscale_000_network} --port=$port \
           --user=$maxscale_user --password=$maxscale_password \
           --logdir=$logdir \
           --test-file=$dir/t/$test_name.test \