There are various tools in the Wormhole ecosystem that can help you in developing xDapps. Here are a few of the most notable:
Wormhole has deployed Core Bridge, Token Bridge and NFT Bridge contracts on various testnets of the chains connected by Wormhole. You can see the deployed addresses here. There's only a single Guardian that oversees the testnets, so you might get a higher rate of missed VAAs than you would on mainnet.
Wormhole Explorer is a tool that will help you parse VAAs after they've been picked up the Guardian network. It's available here.
If you'd like to try out Portal Bridge on Testnet, there's a UI you can use to attest and transfer tokens for testnet, hosted here.
Tilt is a Kubernetes based tool that runs a copy of every chain along side a guardian node to create a simulated testing environment. To set it up and test against it, start here.