test: assert that app exited without signal (#74087)
we've seen these tests flake with this error:
```
expect(received).toBe(expected) // Object.is equality
Expected: 0
Received: null
234 | // App finally shuts down
235 | await appKilledPromise
> 236 | expect(app.exitCode).toBe(0)
| ^
237 | })
238 | })
239 | }
at Object.toBe (production/graceful-shutdown/index.test.ts:236:
```
this PR doesn't fix that, but it should make it easier to see why the
code was `null` -- if signal is nonnull, we'll see what it was.
note: `once(app, 'exit')` returns the arguments passed to the `exit`
callback, which are `(code, signal)`
