I'm working on a simple dApp in AION and I need to have some aion coins to test with. Is there a faucet like the Ethereum Rinkby faucet?

Welcome to StackOverflow Peter! As DavOS pointed out, there's a faucet for the Mastery test network available. Here are the basic steps taken from the Aion documentation.

1. Sign up or login to Gitter.
2. Search for mastery_faucet and join the Aion Mastery Faucet channel.
3. Paste your public address into the channel.
4. A few seconds later the faucet-bot will reply saying your request has been processed.
5. You should now have 1 AION in the address you supplied.

Keep in mind that you can only ask for 1 AION per day from the master_faucet bot. There are a few kinks in this process at the moment. There is a faucet website planned to come out later this year apparently.

Edit

This answer is now out of date. Tipeveryday's answer is correct and should be marked as the correct answer by OP.