Fix: tests: discarded notifications: missing `list-triggers` call

Fix: tests: discarded notifications: missing `list-triggers` call

I get the following test failure when running the
test_notification_notifier_discarded_count test case.

  # Kernel event notifer error counter
  ok 1 - Add trigger my_trigger
  ok 2 - No discarded tracer notification message
  ok 3 - Trigger 'errors: none' notification message
  ok 4 - Tracer notification discarded line printed
  ok 5 - Discarded tracer notification number non-zero as expected
  ok 6 - Remove trigger my_trigger
  ok 7 - No `on-event` kernel notifier enabled as expected
  ok 8 - Add trigger my_trigger
  ok 9 - No discarded tracer notification message
  not ok 10 - Trigger 'errors: none' notification message
   #   Failed test 'Trigger 'errors: none' notification message'
   #   in tests/regression/tools/notification/test_notification_notifier_discarded_count:test_kernel_notifier_discarded_count() at line 108.
  ok 11 - Remove trigger my_trigger

This happens because we test the `list-triggers` output on outdated
content. We need to call `list-triggers` right before testing the output
of it.

In this commit, I also changed the number of iteration to 2000 as it's
sufficient to generate discarded notifications.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I66d54f9178f9f37ff2fd899278e77fe50e455c10