[K2] Add an integration test for jvm-only project

[vmishenev]

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For the JVM integration testing, we have only small projects (it-basic, it-collector). We need to choose a multimodule JVM-only project to test K2 without MPP.

Also, it can dump a Documentable model to compare.

[vmishenev]

Also, it would be nice to have Kotlin/Java mixed project. For example, it could help to find a recent bug with concurrency https://github.com/Kotlin/dokka/pull/3143.

[IgnatBeresnev]

This is only useful if we're going to compare HTML or the documentable model.

[whyoleg]

With the implementation of DGPv2, new integration tests were added, including ones that compare HTML output (#3821). If needed, new tests might be added using the same technique.