Tests: utils: lttng_pgrep spams output when racing with a process

Tests: utils: lttng_pgrep spams output when racing with a process

lttng_pgrep is often used to check if a process is alive. As such, it is
often used on PIDs which are tearing down.

The file redirection used in `tr '\0' '\n' < /proc/"$pid"/cmdline` often
fails (which is correct) because the /proc/$pid folder no longer exists.

When this occurs, the test output is cluttered with annoying errors like:
  ./tests/regression/kernel//../../utils/utils.sh: line 151: /proc/845/cmdline: No such file or directory

This part of the command now runs under a subshell to hide the error
when it occurs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6a26cb63cd56c46557a73e2e475b0cac729cc67f