[themulticaster]

Isabelle/HOL has Quickcheck, which is precisely what you think it is. Although it only works if the free variables in the thesis you'd like to prove are sufficiently simple AFAIK (think integers or lists). The more powerful alternative to Quickcheck is Nitpick, which is a purpose-built counterexample finder.

The workflow you'd like already exists one to one in Isabelle: You can simply type "nitpick" halfway through a proof and it will try to generate a counterexample in that exact context.