test/py: gpt: Use long options for sgdisk

sgdisk 0.8.10.2 from AOSP doesn't support short options, failing with
errors like this:

    sgdisk: invalid option -- 'U'

Test fails due to that error. Let's use long options to make the test
work with any sgdisk version.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
1 file changed