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/cache_basic.sh

@@ -18,7 +18,8 @@ function run_test
     echo $test_name
     logdir=log_$test_name
     mkdir -p $logdir
-    mysqltest --host=${maxscale_000_network} --port=$port \
+    mysqltest --no-defaults \
+              --host=${maxscale_000_network} --port=$port \
               --user=$user --password=$password \
               --logdir=$logdir \
               --test-file=$dir/t/$test_name.test \