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.
By default only E* and F* lints are enabled. I previously followed the
CONTRIBUTING.md instructions to implement a TID* lint and was confused
why my lint wasn't being run.
