Update test-http-set-cookies to use Countdown

[Leko]

Use Countdown class instead of nresponses in test-http-set-cookies.js

See also: #17169

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test

[apapirovski]

Hi @Leko, thanks for working on this — there already appears to be a PR that's approved and ready to get merged dealing with the same test: #17504

For the Countdown conversion tests, I would recommend searching the name of the file within PRs first as there has been a lot of progress on that end and the list in the referenced issue is outdated.

[Leko]

@apapirovski Thank you for information.
I closed this PR.

I would recommend searching the name of the file within PRs first

I got it.