Fix: bt2: pass _TraceClassConst to destruction listeners

Fix: bt2: pass _TraceClassConst to destruction listeners

Trace class destruction listeners currently receive a _TraceClass.  I
think it's a mistake, in that they should receive a _TraceClassConst.
Fix that, tweak the tests to test it.  Test the same thing for trace
destruction listeners (they didn't suffer from the same issue).

Change-Id: I4651ffc44c916d64f6e6f2944fa856b4ed5bb5f6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11447
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>