tests/block_cloning: rename and document get_same_blocks helper

tests/block_cloning: rename and document get_same_blocks helper

`get_same_blocks` is a helper to compare two files and return a list of
the blocks that are clones of each other. Its very necessary for block
cloning tests.

Previously it was incorrectly called `unique_blocks`, which is the
_inverse_ of what it does (an early version did list unique blocks; it
was changed but the name was not). So if nothing else, it should be
called `duplicate_blocks`.

But, keeping the details of a clone operation in your head is actually
quite difficult, without the additional overhead of wondering how the
tools work. So I've renamed it to better describe what it does, added a
usage note, and changed it to return block indexes from 0 instead of 1,
to match how L0 blocks are normally counted.

Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Reviewed-by:  Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15181