I want this for being able to write reasonable expect-test style tests
for oneliners. We will still probably want something like insta for more
complicated test cases where you actually *want* the output in a
different file, but for now this will do.
cc: https://git.lix.systems/lix-project/lix/issues/595
Change-Id: I6ddc42963cc49177762cfca206fe9a9efe1ae65d
