is there something like the paypal sandbox test accounts for coinbase that I could use to test the app that I'm developing?
If you are integrating with bitcoin directly, then you can use the bitcoin testnet (https://en.bitcoin.it/wiki/Testnet).
If you are using a payment processor, Coinbase does not have sandbox feature like PayPal, although they have expressed interest in creating one based on bitcoin's testnet. Coinbase does allow sending test payment notifications (IPN) from their merchant settings page (https://coinbase.com/merchant_settings).
Bitpay's FAQ (https://bitpay.com/faq) states that they too do not have a sandbox, and advises testing with small amounts of BTC.
Somewhat yes. Bitcoin has a Testnet that is a fully functioning Bitcoin network (you can even mine on it) but isn't really traded for money. It's safe to use and send transactions to.