iochannel-test: Fix leaks in test

https://bugzilla.gnome.org/show_bug.cgi?id=711751

tests/iochannel-test.c

@@ -165,5 +165,8 @@ gint main (gint argc, gchar * argv[])
 
     test_small_writes ();
 
+    g_free (filename);
+    g_string_free (buffer, TRUE);
+
     return 0;
 }