Unit Test: Mock, instead of using proxy

[boholder]

Mockito has a way to create an instance of a class without calling its constructor. Let's see if we can delete shallow proxies and mock minecraft code directly.

https://github.com/FabricMC/fabric/blob/1.20.4/fabric-data-attachment-api-v1/src/test/java/net/fabricmc/fabric/test/attachment/CommonAttachmentTests.java#L81