fix: test_trimmer on macOs and Solaris

The 'wc' tool on BSD based system adds leading spaces to it's output,
which breaks the quoted compare. Use bash parameter expansion to trim all
spaces from the 'num_events' variable.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I884f59cf1fcdcb33419dd02c69886715ce4db71b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1654
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>