Tests: Use `_run_lttng_cmd` instead of calling `$LTTNG_BIN` in utils

Tests: Use `_run_lttng_cmd` instead of calling `$LTTNG_BIN` in utils

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

Some commands aren't logged (e.g. `lttng_remove_trigger`) when the
verbose client environment variables are set.

Solution
========

Always pass by the `_run_lttng_cmd` helper.

Known drawbacks
===============

None.

Change-Id: I22c6c93fc9397f9d8ded5ce385ee1fa825fe1cb8
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>