There is also signet, which is a different testnet that works more like Liquid where blocks aren’t mined by hashing, instead they’re just signed by a set of known keys. Mutinynet is another signet.

Arguably signet is better for testing Lightning as the blocks are more predictable and shenanigans like Lopp’s attack can’t happen - testnet is useful for testing anything that needs the chain to behave more like actual Bitcoin (mining software, ASICs, reorgs etc)