test: utils: lttng_pgrep performs lookup on non-existing pid

Observed issue
==============

 # Killing (signal SIGTERM) lttng-sessiond and lt-lttng-sessiond pids: 20962 20963
 ./tests/regression/tools/trigger/start-stop//../../../../utils/utils.sh: line 103: /proc/20963/cmdline: No such file or directory

Cause
=====

lttng_pgrep performs a two step search/validation for the pattern. Since
lttng_pgrep is used during tear-down of process (staged termination
signalling) a process returned by pgrep might exit before the second
check.

Solution
========

Simply silence the error. The code flow already acknowledges the
possibility of failure here.

Known drawbacks
=========

None

References
==========

Fixes: #1292
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I82cb9fd4754c10a5104af495a8a959f4fbd92664