test/py: use absolute dts path in vboot test

Without this, the test fails if the test is run with a cwd other than the
root of the U-Boot source tree.

Fixes: 8729d582595d ("test: Convert the vboot test to test/py")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed