Layers that add additional recipes should ideally be able to add associated tests. The testimage class currently looks in a fixed path for test modules; it would be very useful if it could look in all configured layers for additional ones. This will be immediately useful for tests relating to the new meta-security layer.

This should be pretty easy to implement - the BBLAYERS variable value contains the paths to all enabled layers; if we just search in tests/runtime/ under each of those paths and then this can be the standard location within a layer for introducing test modules.