test: dm: Add test cases for FWU Metadata uclass

Add test cases for accessing the FWU Metadata on the sandbox
platform. The sandbox platform also uses the metadata access driver
for GPT partitioned block devices.

The FWU feature will be tested on the sandbox64 variant with a raw
capsule. Remove the FIT capsule testing from sandbox64 defconfig --
the FIT capsule test will be run on the sandbox_flattree variant.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>