The fixture only needs to mock out the __call__ method, and it needs
to use a real function as the replacement so that in the follow-up
patch we can introspect the result to see if we need to pass the rule
argument to it.
Change-Id: I22b0fc6bb3e40b54a24d9d53c9a20731af0064ee
Signed-off-by: Doug Hellmann <doug@doughellmann.com>