test/py: gpt: update size of gpt partition

- avoid disturbing 0MiB partition size (in fact < 1MiB)
- test overlap limit between part1 and part2
- test gpt write with data with modifier 'M' for MiB

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>