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.
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