Add test for re-defining an identical label

When merging one device tree over the top of a previous tree, it is
possible to define a duplicate label that has the same name and points
to the same property or node. This is currently allowed by the duplicate
label checking code. However, alternative duplicate label checking
algorithms might not allow this. Add an explicit test to ensure this
capability is maintained.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>