test/py: Correctly restore the DT after capsule tests

test/py: Correctly restore the DT after capsule tests

Some capsule tests are changing the sandbox DT to test various features,
e.g authenticated capsule updates, versioning support etc. However, no one
restores the original DT and the CI pops errors looking like

/u-boot
Bloblist at 100 not found (err=-2)
Failed to find FDT file '/tmp/sandbox/persistent-data/scratch/EFI/CapsuleTestData/test_ver.dtb'
initcall_run_f(): initcall fdtdec_setup() failed

if sandbox is restarted.

So let's restore the proper DT after done with the capsule testing.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>