I never saw this package mentioned in the slog proposal or elsewhere. Hopefully this isn't too late.

Test helpers should be with/near their packages, not put underneath the testing package. This makes these helpers more discoverable and easier to understand.

Assuming the idea of a new package for just this one function is still open to debate, instead of the above, I propose this package be merged into log/slog. A single function doesn't merit its own package, in my opinion. Test helpers belong with the code they're helping out with.