XFAIL'd test: alltogether (and other allexe-emitting tools): 'overwrite' behavior for existing but empty files

[dtzWill]

Test for this already in-tree, overwrite-empty.test.

---

I'm not really sure what the desired behavior for overwriting empty files should be (is anything of value lost?),
but the fact that our current code makes it difficult to differentiate between empty and not-existing is unfortunate.

cc #3