test/py: Allow tests to control the sandbox device-tree file

test/py: Allow tests to control the sandbox device-tree file

Normally tests will run with the test.dtb file designed for this purpose.
However, the verified boot tests need to run with their own device-tree
file, containing a public key.

Make the device-tree file a config option so that it can be adjusted by
tests. The default is to keep the current behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>