[buildbots] False pass results for segmentation fault in test

[Markus Mützel]

It looks like a segmentation fault in the last step of running the test suite 
on the build bots is still marked as "success" if there were no failed test up 
to that point. E.g.:
http://buildbot.octave.org:8010/#/builders/12/builds/1525/steps/6/logs/stdio

Maybe any output to stderr in that step should result in it to be marked as 
"failure"?

Markus