port_options.js isn't testing what it thinks it's testing.
It sends SIGTERM before mongod has installed its sighandlers, so mongod dies with that signal. But if mongod has installed its sighandlers, it shuts down normally from the SIGTERM and we should expect clean shutdown. Gabriel has suggested watching the mongod output stream to determine when it's installed its signal handlers, parsed its command options, and is ready to be killed.

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Originally read:

Fix port_options.js to deal with a child process dying with SIGKILL.
Currently it only expects SIGTERM. If a SIGKILL happens to the child for some reason, that's unexpected. But what we should do in that case is die, not fail an assert.

This addresses BF-7447, which is happening too often.