Tests: cleanup: rework trigger listing tests

- Rename `list_triggers()` -> `list_triggers_matches_ok()` to make it
  obvious it's testing something,
- Add `diag` to all test cases,
- Launch only one lttng-sessiond for all test cases to reduce test
  runtime, and
- remove-trigger all triggers at the end of every test cases.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Icbbd08f521565ef1cae901bb192cf6f8f1cdb7ea