functional tests: add pre-start hook test
functional tests: add pre-start hook test

It tests that the pre-start hook runs and that it runs as root even when
the app itself runs as an unprivileged user.

coreos/rkt,joshix/rkt,lucasallan/rkt,kinvolk/rkt,lucasallan/rkt,kinvolk/rkt,lucasallan/rkt,rkt/rkt,alban/rkt,kinvolk/rkt,rkt/rkt,coreos/rkt,alban/rkt,joshix/rkt,coreos/rkt,joshix/rkt,lucasallan/rkt,kinvolk/rkt,rkt/rkt,rkt/rkt,alban/rkt,joshix/rkt