tests: add OP-TEE test suite

OP-TEE can get supplied with a devicetree and will then insert
its firmware node and reserved-memory sections into it.
As this devicetree often is not the one supplied to a later
loaded kernel, a previous commit added functionality to transfer
these nodes onto that new devicetree.

To make sure this functionality stays intact, also add a test
for the transfer functionality.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>