TESTS: Don't use exec for testing stdin. Modify process.stdin directly. Fixes #547

@benesch hey, since you can just do `process.stdin.emit('data', ...)`, do we still need the timeout of half a second?

Seems like two things are going on here: 

1. The timeout simulates a delay in data buffering
2. The timeout is stated to be used because of the lack of being able to trigger two 'data' events. I don't believe this is the case.

If it turns out that case 2 is no longer valid, and if you agree, I will decrease the timeout to avoid unnecessarily slowing down the suite.