Is there a Bitcoin testnet that is really small so you can quickly download the blockchain and test stuff?
Depending what are trying to achieve you can use testsnet box https://github.com/freewil/bitcoin-testnet-box, it runs two nodes one connect to another in our machine without need to access or the testnet, also gives the ability to generate blocks whenever you want so you don't need to wait.
You can also use regtest mode. It creates a new blockchain in a local environment and you can mine blocks on demand. Ideal for unit testing.