Declare normalization_id on file_test_mod2

[rcannood]

Describe your changes

run_benchmark/main.nf extracts metadata from --input_test_mod2 and then does:

def rna_norm = state.dataset_uns_mod1.modality == "GEX"
  ? state.dataset_uns_mod1.normalization_id
  : state.dataset_uns_mod2.normalization_id

rna_norm decides which methods run on which dataset (the preferred_normalization filter), so in the swap direction the whole routing hangs off normalization_id in test_mod2. file_test_mod1.yaml declares that slot; file_test_mod2.yaml never did. It works today only because process_dataset happens to copy it onto both objects.

One-slot change to write down what we already depend on.

Part of a series of PRs coming out of a pre-run review of the benchmark.

Checklist before requesting a review

• I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • Breaking changes
  • New functionality
  • Major changes
  • Minor changes
  • Bug fixes

• Proposed changes are described in the CHANGELOG.md

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