tests: make `bt_diff()` able to write expected files

tests: make `bt_diff()` able to write expected files

Make `bt_diff()` copy the "actual" file over to the "expected" file, if:

 - the comparison between "actual" and "expected" failed
 - the "expected" file is a regular file
 - the `BT_TESTS_DIFF_WRITE_EXPECTED` env var is set and not `0`

This makes it much easier to update `.expect` files than doing it by
hand.

Change-Id: Ia55b55035591d521c627b1022d6dcbde9ed541a7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13313
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>