test: stop the GET rate-limit test flaking on a minute boundary

[snekxs]

What

The GET rate-limit test fired 241 requests in a tight loop and asserted the 241st returned 429. It depends on the wall clock.

functions/_middleware.js buckets requests as:

const key = `window:${ip}:${method}:${Math.floor(Date.now() / 60_000)}`;

If the clock ticks into the next minute partway through the loop, the counter moves to a fresh bucket, the cap is never crossed, and the loop finishes on 200 — the test then fails depending only on when it happened to run. That is why it passed on retry both times it was seen.

Fix

• Pin Date.now to a fixed instant inside a single minute so the run cannot straddle the boundary. Behaviour is unchanged (cap is 240 GETs/min, the 241st crosses it); only the clock is frozen, and it is restored in a finally.
• Add the GET rate limit resets on a new minute, which moves the clock deliberately and proves the reset — turning the race the old test tripped over into an asserted behaviour.

Verification

• Reproduced the flake first: with a clock that advances ~100ms per call, the old loop ends on 200 instead of 429.
• Fixed suite: 12/12 clean runs.
• Full suite: 266 tests, 0 failures; tsc --noEmit clean.

Test-only change; no production code touched.

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