Are Bitcoin and Litecoin test addresses compatible? It seems to me that they are. I mean you can send test bitcoins to test Litecoin addresses. I don't think it's problem at all, but it can cause some confusion. 

Real addresses cannot be mixed, but I managed to send test coins from Litecoin wallet to Bitcoin test wallet. The coins did not come obviously.
Yes, they're interchangeable.

The components of an address are a prefix byte, a 20-byte public key hash (160 bits), and a 4-byte checksum.  Litecoin testnet uses the same prefix byte as Bitcoin testnet (hex 0x6f, decimal 111) and computes the hash and checksum in the same way.  This was arguably not a good decision on the part of the Litecoin developers, but we are sort of stuck with it now (maybe until testnet4).

If you have the private key for the address in your Bitcoin wallet client, it should in principle be possible to export it from there and import it into your Litecoin client (they also both use the same prefix byte for testnet private keys).