tests/lib: use `sink.utils.dummy` in `utils/run-in.cpp`

tests/lib: use `sink.utils.dummy` in `utils/run-in.cpp`

Instead of implementing another dummy sink.

Note that when executed manually, test programs using the `runIn` util
(e.g. `test-fields-bin`) must be executed through
`tests/utils/run-in-py-env.sh`, for the `utils` plugin to be found.

Change-Id: Ib40480089c7dea35fddc7144b0c99873723f8fe8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11824
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>