Support tracking valid cases

[CodeLenny]

If a generator can't be modified, often test cases will need to be thrown out. Add an API for tracking run metrics, and testing run statistics in the master test.

Something like:

const jsc = require("jsverify");
jsc.ava = require("ava-verify");

jsc.ava({
  needCases: {
    notZero: 10, // or "10%"
  },
}, [ jsc.int, jsc.int ], (t, a, b) => {
  if(b === 0) {
    t.context._avaVerify.case("notZero");
    return t.pass();
  }
  t.true(a / b < a);
});

Then if notZero disables more than 10 of the runs, a test in the master test would throw an error.