Change the way we handle VM image in tests

[lstocchi]

This is an idea suggested by @cfergeau to improve how we use VM images during tests on macOS.
> I could change it by deleting the compressed file and keep the original uncompressed file.... and copy/paste it to be used for every new tests run. Would it be ok? Or you have another idea? We would waste some disk space but we would be safe as tests will always run on a clean image and instead of uncompressing we will just copy it (should be faster??)

On macos, you can use unix.Clonefile with the uncompressed disk image, and throw away that cloned file at the end of the test. This should be fast and only use disk space for changed blocks.

Originally posted by @cfergeau in #427 (comment)