Print out stdout and stderr of tests on SIGTERM

[sunshowers]

While attempting to diagnose oxidecomputer/omicron#4779, we found it hard to do so because nextest does not show the output of running tests on SIGTERM.

We don't print out the stdout and stderr of tests on SIGINT, because it is quite ugly to do so on Ctrl-C (though maybe we should write stdout and stderr to a file). However, SIGTERM almost always means an automated system like a CI timeout killing nextest, so we should print out the output in that case.