Add configuration option to allow unreachable! in test functions

[AMS21]

changelog: [unreachable]: Add configuration option to allow usage in test functions

Summary Notes

• Feature-freeze by samueltardieu

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

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[samueltardieu]

Seems that you are trying to add a new configuration option!

We are currently in a feature freeze, so we are delaying all lint-adding PRs to September 18 and #15086.

Thanks a lot for your contribution, and sorry for the inconvenience.

@rustbot note Feature-freeze
@rustbot blocked
@rustbot label +A-lint

[dswij]

r? clippy

[Alexendoo]

What's the motivation here? In my view the combination of a restriction lint & using it enough in tests that a configuration option is better than a one off allow seems fairly niche