SELFOUR-242: port fastpath test to slow path (possibleSwitchTo)
SELFOUR-242: port fastpath test to slow path (possibleSwitchTo)

Unless the tests match directly, the fastpath proof becomes hideous.
Upside to this is that slow path now skips scheduler enqueue only to
dequeue the same thread a moment later.

cmr/seL4,cmr/seL4,cmr/seL4