[FEATURE] Having a langchain4j-test module with helper classes

[agoncal]

Is your feature request related to a problem? Please describe.

Today we have test classes (such as ChatModelMock) that would be helpful to expose so we can use them in our unit/integration tests. It would be nice to have a separate langchain4j-test module that we could use.

Describe the solution you'd like

Have a separate langchain4j-test module, have langchain4j-core depending on it.

[agoncal]

After a quick analysis, ChatModelMock implements ChatLanguageModel. If we move ChatModelMock to a langchain4j-test module it will depend on langchain4j-core module... which will depend on langchain4j-test. This will break because of cyclic dependency.

I'll investigate a bit more