Ensure multiple cookies set in dev result in multiple set-cookie headers

[matthewp]

Changes

• Using res.setHeader which accepts an array of headers, which results in separate header lines. This is required for set-cookie.
• Fixes Can't set multiple cookies in response on v2 #6525

Testing

• Updated an existing test. Using raw sockets so that we can examine the raw response and see that there are 2 separate set-cookie headers. You can't examine this for sure using fetch because you can't see the raw response.

Docs

N/A bug fix

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[Princesseuh]

The test that failed is related to the change here. Since all other pipelines passed, I'll just rerun it to see if it's flaky, but due to undici differences between versions I wouldn't necessarily be surprised if there's a real change