tests: kill output message change

Commit 09af3db48e01e5744f15fd8a2395e0fed36526ea changed how a missing
process is reported, which caused kill test to fail when the test suite
is ran as root.

Reference: https://travis-ci.org/karelzak/util-linux/jobs/31312170
Signed-off-by: Sami Kerola <kerolasa@iki.fi>