Add regression test for complex friendships logic using RNTupleProcessor

[matildamarjamaki]

GH20033: Add regression test for Create/CreateChain joins

Changes

• Add a regression test for GH20033.
• Convert the test into a parameterized test to cover all combinations of RNTupleProcessor::Create and RNTupleProcessor::CreateChain for the joined processors.
• Add a SameFieldName test to verify correct field namespace resolution when all joined RNTuples contain a field named value.
• Add proper cleanup of temporary ROOT files in TearDown().

Checklist

• tested changes locally

Local testing:

• Built ntuple_processor
• Ran all 32 parameterized test instances successfully.