add pytests for 'gpt rename' and 'gpt swap'

Add unit tests for the 'gpt rename' and 'gpt swap' commands that
rely on the block device created by test/py/make_test_disk.py.
Add CONFIG_CMD_GPT_RENAME to the sandbox_defconfig.  Remove the
testdisk.raw test device at the end of the tests.

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