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
Differential Revision: https://reviews.freebsd.org/D47005