How to set up a proxy test framework?

Thanks everyone for your feedback during today's versioning talk 
<https://datatracker.ietf.org/meeting/120/materials/slides-120-httpbis-versioning-for-http-resources-00.pdf>!

On slides 10 and 11, Mark Nottingham keenly pointed out some issues 
we'll encounter with proxies. I want to set up a test framework to help 
find and validate a solution. Has anyone set up a such a proxy test 
framework before?

I think my first step should be to list some scenarios or softwares to 
test with. I don't have a lot of experience yet in proxyland. I'd love 
advice!

Thanks!

Michael