test/py: test_ut.py: Ensure we use bytes

In the case of some unit tests we are working with providing a fake
flash device that we have written some text strings in to.  In this case
we want to tell Python to encode things to bytes for us.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>