tests/utils/utils.sh: bt_diff(): validate that both files exist

tests/utils/utils.sh: bt_diff(): validate that both files exist

Otherwise bt_diff() returns 0 (happy) with a nonexistent file.

This change reveals that `tests/plugins/src.ctf.fs/field/test-field.sh`
passes, but for the wrong reason: run_python() doesn't exist (I replaced
it with bt_run_in_py_env() a while ago), therefore that line doesn't
create any expectation file and bt_diff() returns 0. Change it to
use bt_run_in_py_env().

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I259123f5c0946ece7f58e4ef185313bf26354b46
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12534
Tested-by: jenkins <jenkins@lttng.org>