Fix stretch itest missing "ps" command, use pipefail to catch it earlier in the future

[chriskuehl]

The test was failing but not generating a nonzero status code because pipefail wasn't on.

Previously it looked like this:

+ exec dumb-init /mnt/tests/test-zombies
+ sleep 1
+ bash -euxc 'bash -euxc '\''echo i am a zombie'\'' &'
+ bash -euxc 'echo i am a zombie'
+ echo i am a zombie
i am a zombie
++ ps -A -o state
++ grep Z
/mnt/tests/test-zombies: line 13: ps: command not found
++ wc -l
+ num_zombies=0
+ '[' 0 -ne 0 ']'

Target itest_stretch completed successfully

(stretch is unreleased and something of a moving target, so it's not totally unexpected that something like this might happen -- but obviously the tests should have caught it!)