XProc testing; mock steps and v.next

So I've been fiddling with testing some XProc steps lately, and realized
that there's no way (afaik) to create a "mock step" without editing either
the step you want to test or the XML catalog. So it's not possible to do
unit testing in XProc; only integration testing.

I think it would be useful to be able to override a URI that is already
defined in a XML catalog. Say I want to test the following step:

<p:declare-step type="x:mystep" xml:base="file:/tmp/mystep.xpl" ...>
    <p:import href="http://www.example.com/mydependency.xpl"/>
    ...
</p:declare-step>

But I don't want x:mystep to invoke
http://www.example.com/mydependency.xplas it might produce
non-deterministic side-effects. What if I could do
something like this to override the XML catalog:

<x:mystep>
    <p:resource name="http://www.example.com/mydependency.xpl"
uri="mockdependency.xpl"/>
</x:mystep>

The Resource Manager page on the v.next wiki[1] isn't very detailed yet but
maybe it will be possible to do something like this with the planned
resource manager?
[1] http://www.w3.org/wiki/XProc_Resource_Manager

(another thing that would make XProc testing easier would be dynamic
pipeline execution; p:evaluate/cx:eval)

Jostein