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]

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