Support running a test function multiple times with different data

You can already do this with macros, but it's a bit verbose and complicated. It would be nice to simplify the common case of just running the same test over an array of fixtures.

Possible API:

```js
test('validate email', t => {
console.log(t.data);
t.true(isString(t.data));
})
.data([
'foo',
'bar'
]);
//=> 'foo'
//=> 'bar'
```

Start by reviewing AVA's existing macro support and test-runner API; the issue does not name specific files or tests. Done means a concise data-driven API runs the same test once per array fixture and makes each fixture available as t.data.

Written by the indexing model from the issue text.