This and other assertion messages in this file need work. You have to imagine you're viewing these in isolation, without the code, just the test name and assertion message.

Try to use active voice too. Instead of "should be thrown" ... try something like

Registering a custom element type name that is already registered should throw an exception

Please start with this and improve on it... the message should relate to the condition being tested, which is that the registries are shared. Integrate something to do with "shared registry" from your original message.