Detect integration tests in is_in_test

[ClementTsang]

Related: #13981

This PR makes it so that the is_in_test check accounts for if a node is in an integration test file (that is, under tests/). This should fix the issue where some clippy settings work for cfg(test) but not for integration tests.

This could be extended to benches as well if this seems reasonable.

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Note that this is the first time I'm contributing to this repo, so please let me know if there's anything to change. I'm not really sure how I was meant to test this for example, especially if I want to test an example of this where it should still "fail", I guess (should I split the integration_test with two crates?).

changelog: [unwrap-used]: allow-unwrap-in-tests works now on integration tests.

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[ClementTsang]

Hmmm it looks like it's causing problems because now some of clippy's tests are being treated as valid under is_in_test... let me set this to draft for now and figure this out. For whatever reason I didn't catch this when testing locally earlier.

[rustbot]

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

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.