elbepack: finetuning: fix losetup usage for command in losetup element

[koalo]

needs_loop_device listed partition-command (which is actually the type of the
element) instead of command. This leads to commands in losetup no longer
having access to a mounted image and thus fail.

Fixes: d2d5925 ("elbepack: finetuning: avoid losetup where possible")

Also add unit test that fails without the bug fix.

[t-8ch]

Thanks!

I picked up the bugfix.
For the new test I dislike that it relies on all kinds of internal implementation details.
If there is no nice way to have a unittest, instead we can add the project finetuning to tests/simple-validation-image.xml and validate the results in tests/simple-validation-image-test.py.