Currently the preview mode work (in Valhalla) is being migrated to mainline and while it is unit tested, it is important to add a high level test which demonstrates it works as expected, regardless of any Valhalla semantics.

In preview mode:
* preview classes must be loaded as expected (either new or alternate versions of existing classes).
* preview resources must be available as expected

In non-preview mode:
* Preview classes/resources should not be visible

This test would have to build a custom runtime with test preview classes/resources in specific system modules. In particular it needs to test boot modules, platform modules and application modules.

It should also test that non-system modules (unknown to the runtime) cannot use preview mode in any way.

The test should be run for both a packages build and an exploded build (perhaps with module patching just to demonstrate that patching still takes priority).