Tests: add `tap` utils code to Black formatter exclusion list

Those files are not written by us but rather copied into the project.
Let's not reformat them automatically so we can compare them with future
versions.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I98ccd8475b5367f1b3ed9084ba06965f28bc6f93
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1686
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>