Why are you using a class variable here?  (Also, what does the name "c" signify?).  Given that all the blocks below are closures you just use a local variable.

All that said, I don't usually spec my shared contexts.  It's fine to write these up as a way to prove to yourself that it works, but I wouldn't keep this spec.  It doesn't add value to rspec-mocks, and is kinda hacky.  If the shared context stopped working, we would know pretty quickly since config changes would leak and break other specs.