This change makes tape work the same with synchronous
and asynchronous functions.

```
test('my test', () => {
  throw new Error('oopsie')
})

test('my async test', async () => {
  throw new Error('oopsie')
})
```

These two cases now have the same semantics which means you
can safely use an async function because the unhandled rejection
will be converted into a thrown exception.

Failing a test when the return promise rejected will fail a test
that was probably silently broken previously.

Extra test cases have been added to reflect real world usage
of tape with async functions which we preferably do not want to
break.

…est.

Also, implicitly call `.end()` if not already called when a Promise is returned, because the promise itself marks the end of the test.