daemon: tests: add a test for missed SIGTERM
This is somewhaht hard to test reliably, but we'll give it a shot. Startup
a sleep(1) daemon with a hefty restart delay. In refactoring of daemon(8),
we inadvertently started dropping SIGTERMs that came in while we were
waiting to restart the child, so we employ the strategy:
- Pop the child sleep(1) first
- Wait for sleep(1) to exit (pid file truncated)
- Pop the daemon(8) with a SIGTERM
- Wait for daemon(8) to exit
The pidfile is specifically truncated outside of the event loop so that we
don't have a kqueue to catch it in the current model.
PR: 277959
Reviewed by: des, markj
(cherry picked from commit 9ab59e900c1dd693b4d14285389a035e81341789)