Namespace the dumped old/new.xml fixture files that are created on spec failure.

Every integration spec failure dumps out a pair of `old.xml` and `new.xml` files. This is fine when there's only one failure, but is not useful when multiple things fail; each failure overwrites the last's files.

This namespaces the files (eg. `policies_edit_2.old.xml`), and updates the ignores and pre-test `rm` that's run on the files from the previous round.