Move test data files further away from code #31

I didn't properly foresee this in issue #14 and when reviewing its PR, but for me the many test files are often getting in the way when working in the split-patch/ directory.

I think we should move them (back) to the top-level test/ directory, although into a separate subdirectory test/data/, and move the existing contents of test/ to test/bin/. I.e. the goal is that whenever one cds into a directory that has code files or is the base dir of a crate, there are no test data files below it--e.g. make "git grep" not report data files when searching for something in the code.

This is not urgent, though.