Add integration test framework

[mezz]

This PR adds basic integration tests to the project, as a foundation for future changes or optimizations.

It's kind of big because I needed to write a bunch of builders to make the tests readable. I tried to add tests for basic functionality, to show that both the current code and the integration test builders are working correctly.

For the review, it's probably best to just read the actual tests for correctness, and then see if there's some obvious flaw in the builder.

I think the main weakness of some of these tests is that they check the output logs, for example to see if things were cached or not. We could probably harden that but for now I think we can just update the tests if we change those log strings.

There is only one real minor code fix in here,
in src/main/java/net/neoforged/neoform/runtime/actions/ApplySourceTransformAction.java to set absolute paths. It is tested with an integration test SourceTransformIntegrationTest #appliesAccessTransformerPaths

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