Report to user when subprocess is killed via signal

[emonty]

Issue description

When one of the stestr subprocesses is killed by a signal (such as what happens with OOMKiller is invoked) - the result is the word "Killed" in the stdout and the appending of _StringException to the test name in the subunit.

An example of html from the subunit of a run where this happened is here:

http://logs.openstack.org/03/592303/23/check/openstacksdk-functional-devstack-tips/64f4c71/testr_results.html.gz

and the stdout output here:

http://logs.openstack.org/03/592303/23/check/openstacksdk-functional-devstack-tips/64f4c71/job-output.txt.gz#_2018-10-02_22_23_03_867658

It's possible in C to detect that a subprocess was killed by a signal, and also what the signal was. It would be nice, if it's possible to do the same in python, to report to the user in some way that the process was killed by an external signal, as otherwise it seems like a test was killed by a hard timeout.