tests: Add helper functions to validate if lttng modules are loaded

tests: Add helper functions to validate if lttng modules are loaded

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

Regression tests exercising kernel modules don't validate before and
after the test execution that there are no LTTng kernel modules
loaded.

Solution
========

Add convenient tap helper functions that may be used by shell TAP
tests.

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

None.

Change-Id: I8244358137c2d049ed72e6dc7d9290cb9dda10e3
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>