This will allow us to control the value from test plans, in particular
for disabling at least some unexpected message checks for reverse
dependency testing. We don't want to disable unexpected messages
in general for fmf, as we are looking for exactly these in e.g.
selinux-policy reverse dependency tests.
Move from `su` to `runtest`, as with the former it's impossible to plumb
through variables with non-trivial characters, as they cannot be quoted.
Taken from c38692fa4c
(cherry picked from commit 9544f57220)
