prove_quickcheck_main should be able to dump a test program for counterexamples

Right now, prove_quickcheck_main dumps the counterexample property. Ideally, for fast interactive testing, it should dump a test suitable for interpreter_main, i.e.

```
#![quickcheck]
fn prop_A(x) -> bool { f(x) }
```

should (if one exists) dump a test using a counterexample

```
#![test]
fn prop_A_counterexample(x) {
assert_eq(f(counterexample), true)
}
```