webdav: factor out a moveFiles function, and make the tests call that
instead of FileSystem.Rename directly.

Dir.Rename's behavior wrt overwriting existing files and directories is
OS-dependent.

Fixes golang/go#9786

Change-Id: If42728caa6f0f38f8e3d6b1fcdda8c2d272080d6
Reviewed-on: https://go-review.googlesource.com/4341
Reviewed-by: Nick Cooper <nmvc@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>