tests: improve debuggability of `cli/test-trace-copy.sh`

When something fails in `cli/test-trace-copy.sh`, we don't really know
why, as everything is sent to `/dev/null`.

Make the test record stderr and print it (using tap's `diag`) when
something fails.

Add the `diag_file` helper to make that easy.

Change-Id: I2eb85cd5fffb136cddf2457089681ee57c21e4f7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11963
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>