test/py: regenerate persistent GPT image if code changes

test_gpt generates a persistent disk image which can be re-used across
multiple test runs. Currently, if the Python code that generates the disk
image change, the image is not regenerated, which could cause test
failures e.g. if a test was updated to expect some new partition name or
size, yet the persistent disk image contained the old name or size. This
change introduces functionality to regenerate the disk image if the
instructions to generate the image have changed.

Signed-off-by: Stephen Warren <swarren@nvidia.com>