tests: bt_diff_cli: compare expected `stderr` too

tests: bt_diff_cli: compare expected `stderr` too

Useful to confirm that a trace is read without warnings but could also
be used to confirm that an expected warning is indeed present.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I0c48c68a0aac832729da2d9c5b849fb58ec578e4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1638
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>