Fix test: don't link tests directly with plugins

Move the debug-info code to a convenience library that is then used by
both the plugin and the test code. This fixes build with static
libraries disabled where it's not possible to link directly the plugin
shared object which is a module.

Reviewed-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>