tests: make bt_diff check a single file pair

tests: make bt_diff check a single file pair

bt_diff currently accepts 4 file arguments:

  - stdout expected
  - stdout actual
  - stderr expected
  - stderr actual

To allow for more flexibility, change it to check a single pair.  Tests
that need to check for both stdout and stderr will need to call it
twice.

Change-Id: I400d251e77931a3a457214fabdc59f4c56be7ce1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1838
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>