pkill: tests: do a pass for case-sensitive conflicts

As in 76b6a59f9d30a5a39a5ed, encode upper-case flag tests with a leading
underbar to avoid collisions (thus, erroneously dirty git repos) on
case-sensitive filesystems.

PR: 270948
Sponsored by: Klara, Inc.

(cherry picked from commit 822057bfbbbcdb722a7ccd0e40d6b7e82b1d83f7)