pkill tests: Fix pkill usage in the pkill -x test

The target process name(s) mark the beginning of the command's
positional parameters, so the -P filter wasn't getting applied as
intended. As a result, the second "pkill -x sleep -P $$" would kill all
sleep(1) processes in the system, which can cause problems when running
tests in parallel.

MFC after: 2 weeks

(cherry picked from commit 57b09e470dbd84d1491a8972cf504b25e788a6c3)