yocto-check-layer: Avoid bug when iterating and autoadding dependencies

If iterating a layer with multiple components and auto-adding dependencies
the tests can break since layers are never removed and order isn't guaranteed
to account for that.

Fix this by resetting the layer list back to the original list each time
before auto-adding the dependencies in each case.

This fixes scanning of meta-openembedded in particular where the sublayers
may not be added in order of minimal dependency.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>