Fix test if "partition" not defined in globals

Use default_partition by default for the test

testsuite/expect/test2.18

@@ -212,6 +212,9 @@ expect {
 	}
 }
 
+if { [string length $partition] == 0 } {
+	set partition [default_partition]
+}
 set node_name [ get_idle_node_in_part $partition ]
 set cluster_name [ get_cluster_name ]