Change the meaning of ts.preview so it just controls the feature packs or dists used when running the testsuite and doesn't control what mechanism is used to provision. Use ts.layers and ts.bootable to control the provisioning mechanism.

As part of this we'll also need to figure out how to disable tests that won't work with preview.

This is partly to lay a foundation for dual-EE mode, which may be analogous to what I see happening with – a profile that selects feature packs or dists.