The Desktop Head Unit (DHU) https://developer.android.com/training/auto/testing/index.html provides a virtual display that emulates the protocol used for Android Auto. You plug your phone in via a USB cable, and the phone thinks the DHU is a real vehicle display. However, generating the display is a very CPU intensive operation requiring 3D acceleration, and the regular Android emulator is not fast enough for this. So therefore it is not supported.
However, if you want to do automated testing, you may not need to have a real vehicle display connected at all, and could therefore use an Android emulator. Android Auto messaging apps use the regular Notification API, so you could do tests to check the Notifications are created correctly. Android Auto media apps use the standard L media framework, and you could do tests to check for this as well.