Fix Rubocop offense for StrongMemoize

Vasilii Iakliushinrequested to merge
406761_correct_rubocop_for_strong_memoize into master

What does this MR do and why?

Contributes to #406761 (closed)

Fixes an offense for Rspec/UselessDynamicDefinition in strong_memoize_spec.rb. No user-facing changes.

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