ava: Add idle timeout support

Just switched to AVA – and I love it! – but, – and this is really a minor issue – I also noticed that I kept reusing code like this:

import test from 'ava';

function failAfter(time, t){
  setTimeout(() => {
    t.fail('timed out after ' + time + ' ms');
    t.end(); /* fail fast! */
  }, time);
}

async function thisShouldntTakeLong(){ /* or does it? >_< */
  return new Promise(resolve => setTimeout(resolve, 9999));
}

test('this shouldnt take long', async t => {
  failAfter(1000, t);
  await thisShouldntTakeLong();
});

And just to be clear, I don’t like global test timeouts – I think they’re a terrible idea.

Yet again, I think something like t.failAfter(time) is not too bad.

What do you guys think? 😃