New lint: redundant_test_prefix

[farazdagi]

This PR has started as an effort to proceed from the feedback in #12861.

• Checks test functions for redundant "test_" prefix.
• Auto-fix is supported (and handles collisions gracefully, see below).
• If removing "test_" prefix from, say, test_foo() results in a collision (either because function foo() is defined within the current scope, or because the foo() call exists within function thus creating an unwanted recursion), lint suggests replacing prefix with suffix, i.e. test_foo() becomes foo_works(). The _works suffix is configurable.
• Integration tests do not have #[cfg(test)] defined for them, so by default they are not checked. There exists an option to make sure that any functions with #[test] annotation (including those outside of nodes marked with cfg(test)) are linted.

fixes #8931

changelog: new lint: [redundant_test_prefix]

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dswij (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[farazdagi]

@dswij any ETA on when you'll be able to review (it has been more than 2 weeks -- so, I am not sure whether there needs to be some action on my part to get this rolling).