test: use awaitable apm.flush() in undici tests

[atharv-sys32]

Closes #3176.

apm.flush() is now natively awaitable (added in #3167), so the promisify(apm.flush.bind(apm)) wrappers in the undici tests can be replaced with direct await apm.flush() calls. This removes the leftover promisify imports too.

Verified locally: both test files pass (undici.test.js 83/83, fetch.test.js 17/17) and eslint is clean.

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💚 CLA has been signed

[atharv-sys32]

Hi @david-luna @trentm, quick note on this one. It closes #3176 by switching the undici tests to the now-awaitable apm.flush() and dropping the promisify wrappers. Both test files pass locally and eslint is clean. Would appreciate a review when you have a moment.

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[trentm]

Thanks!