tests/rtt: add a dummy rtt to load_attachments test
tests/rtt: add a dummy rtt to load_attachments test

This will allow to test the fix described by
3df024fa21e241f2efd2ee42b179f4d568f8414e when rendering this scene
to a texture.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl