Tests: test_exclusion: exclusion after tracing active

This testcase tests for a UST bug where exclusions made when the tracing
is active are not excluding the undesired events.

This UST bug was fixed by the following commit:
  commit de713d8a77cbd77e60f58537b0fc222f98fde395
  Author: Francis Deslauriers <francis.deslauriers@efficios.com>
  Date:   Tue May 5 11:51:58 2020 -0400

      Fix: event probes attached before event enabled

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6d896682d5f5e16582ab195c6f4d6946de394843
Depends-on: lttng-ust: Id984f266d976f346b001db81cd8a2b74965b5ef2