const responses = requests.map(() => makeRes());

const runs = requests.map((req, index) => handler(req, responses[index]));

await new Promise((resolve) => setTimeout(resolve, 0));

// Default maxInFlightPerKey is 8; 12 total requests leaves 4 rejected with 429.

expect(countMatching(responses, (res) => res._status === 0)).toBe(8);

expect(countMatching(responses, (res) => res._status === 429)).toBe(4);