tests: add kcmp-y test

tests: add kcmp-y test

It is not very useful as of now, but it will allow tracking changes
in fd output once they are implemented.

* tests/gen_tests.in (kcmp-y): New test.
* tests/kcmp-y.c: New file, wrapper for kcmp.c
* tests/kcmp.c: Add opening of some files for which info may be printed
in the future.
* tests/pure_executables.list: Add kcmp-y.
* tests/.gitignore: Likewise.