tests: kern: add some tests for recently added logsigexit

We don't bother tweaking the sysctl in these tests, we'll just try with
it forced both on and off via proccontrol(1). This could be problematic
in the face of pid wrapping if we got really unfortunate, but the
potential solutions need careful consideration- you probably don't want
to assume a certain velocity of messages into syslog, so just checking
the last N lines is probably similarly flakey.