fix: test_trimmer on macOs and Solaris

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: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I15aef8c6987e40bcb018f32c7c094eabbc3d83c3
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1476
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>