tests: Add Makefile to build debug-info test artifacts

tests: Add Makefile to build debug-info test artifacts

The current documentation on building the debug info test artifacts is
missing some important details, add a Makefile to make the process more
repoducible.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I188ce56bbe4d446e8959e18d2f4c50b8a82a4cb0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1343
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>