tests: update test_trimming to remove the wrapper

Run babeltrace with the python environment set in bt_diff_cli instead
of wrapping the test script in another script. Also fix some related
shellcheck errors in test_trimming.

Change-Id: Ie465dfd65c15a0e51f9474e7d3ebe693a2308488
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1657
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>