add liveslots tests of HandledPromise handler handoff

[warner]

In @FUDCo 's PR for #766 we're seeing sharing-service test failures that probably have to do with pipelined messages causing liveSlots to attempt to re-use a retired Promise, probably because a HandledPromise closed over that vpid in one of its handlers. We believe the Presence's associated HandledPromise (the second HP) was supposed to have displaced that first HP, at least it should have replaced the handlers therein.

The task here is to add a unit test to test-liveslots.js that imports a promise, resolves it to a Presence, maybe gives it a turn to finish resolution, and then sends a message to the original promise. The syscall.send() thus created should be aimed at the presence, not the promise.

[warner]

Done. The test fails (and is thus currently disabled), and the fix will happen under ticket #823