refactor: use structured binding instead of first/second in processor_tests
refactor: use structured binding instead of first/second in processor_tests

Summary: more readable, less code. fix a typo while at it.

Test Plan:
  ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9886

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