chat: RIP modules, long live gosh+v23test
chat: RIP modules, long live gosh+v23test

Also, per discussion:
- Moves 'v23test' package to x/ref/test/v23test
- Makes 'expect' package explicitly depend on 'testing', now
that 'modules' doesn't pass it an interface

MultiPart: 5/7

Change-Id: Idd9624eacb0ddc2d73c6ae5716ec0cf29b241ce6

vanadium/chat,vanadium/chat,vanadium/chat,vanadium/chat,vanadium/chat