Avoids translating a new signal while suspended, as the resulting lock acquisitions can lead to deadlock. An incoming signal there should always be delayable in any case: the only reason we were trying to deliver immediately was due to a too-broad check put in for #2659 (closed).
It is not easy to create a test that reliably exercises these races: our existing detach_spawn and other tests are the closest and this should reduce their flakiness.
Fixes #3026 (closed)