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e9ec2a7b - Add `use test_case::test_case;` to CONTRIBUTING.md

Add `use test_case::test_case;` to CONTRIBUTING.md I previously tried adding the #[test_case()] attribute macro and got confused because the Rust compilation suddenly failed with: error[E0658]: use of unstable library feature 'custom_test_frameworks': custom test frameworks are an unstable feature which is a quite confusing error message. The solution is to just add `use test_case::test_case;`, so this commit adds that line to the example in CONTRIBUTING.md to spare others this source of confusion.