Tests: add hidden trigger visibility test

Tests: add hidden trigger visibility test

Add a regression test for the previous commit that verifies that
internal triggers used by the session daemon to implement various
features (automatic session rotations based on their consumed size, in
this instance) are not visible to users of liblttng-ctl.

The test is written in C to use the library directly. This is needed
since the `lttng` client filters-out anonymous triggers and thus, would
not allow us to see those triggers since they are anonymous by default.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1b8fca648953b8cba49a9888593b3486457d01b2