Make various changes to the GPT test:

1) Reference the disk image using an absolute path in all cases. This
allows test/py to operate correctly if it's run from a directory other
than the root of the U-Boot source tree.

2) Store the disk image in the teswt/py persistent data directory. This
removes the need to re-generate it every time the tests are run.

3) Execute sgdisk using u_boot_utils.run_and_log() so that its output is
captured in the test log. This allows debugging any problems running it.

4) Make the disk image a test fixture. This removes the requirement to
always run all GPT tests, and run them in order. The current code doesn't
create the disk image if e.g. just test_gpt_uuid() is executed via the
test.py -k command-line option.

5) Use @pytest.mark.buildconfigspec for all feature dependencies, rather
than manually implementing some of them.

6) Make all tests depend on sandbox, since they use the sandbox-specific
host command.

Fixes: a2f422555fc8 ("add pytests for 'gpt guid' command in sandbox")
Fixes: c5772188ede9 ("add pytests for 'gpt rename' and 'gpt swap'")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Tom Rini <trini@konsulko.com>

Provide a Python function that creates a small block device for the
purpose of testing the cmd/gpt.c or cmd/part.c functions in the u-boot
sandbox.

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