add pytests for 'gpt guid' command in sandbox

Run unit tests for the 'gpt guid' command, making use of the block
device created by test/py/make_test_disk.py.  Remove this device at
the end of the tests.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>