Nice to have a "local loopback" bus state

### Is your feature request related to a problem? Please describe.
It would be nice to be able to request local loopback mode (for adapters that support it) using a standard API rather than a side-channel based on adapter-specific knowledge.

Local loopback (when supported) is a useful testing tool.

### Describe the solution you'd like
Something along the lines of a LOCAL_LOOPBACK entry in the enum.

Ideally interfaces already reject states they don't support, but perhaps this would also require interfaces to learn to be more picky about state requests?

### Describe alternatives you've considered
Obviously the alternative is private methods on the interface, which seems to be the standard way of dealing with per-interface weirdness already.