fx115: Scaffold: Fix Tests listbox sizing

- Remove splitter for now - no longer works with flex, so we'll need to
  rethink it
- Borrow selectItemsDialog richlistbox sizing fix via mixin

scss/components/_selectItemsDialog.scss

@@ -42,7 +42,5 @@
 // unless all/most elements in the hierarchy that contain that
 // richlistbox have a min-height: 0 set
 .contain-richlistbox {
-    vbox, hbox {
-        min-height: 0;
-    }
-}
+    @include contain-richlistbox;
+}