When using rts_setInCallCapability, lock incall threads

This diff makes sure that incall threads, when using `rts_setInCallCapability`, will be created as locked.
If the thread is not locked, the thread might end up being scheduled to a different capability.
While this is mentioned in the docs for `rts_setInCallCapability,`, it makes the method significantly less useful as there is no guarantees on the capability being used.

This commit also adds a test to make sure things stay on the correct capability.