test(ring): reduce wall time of slow tests by ~42%

[colega]

What this PR does:

CI splits unit tests into 3 parallel groups but the ring package alone takes ~10m42s under -race — 78% of total test time — pinning group 0 at ~13min while groups 1/2 finish in 1–2min.

Four tests dominate that ring runtime (321s of 492s locally). All run very large inputs that race detector inflates without adding correctness coverage. This PR shrinks those inputs and removes redundant work:

• TestDoBatchWithOptionsContextCancellation builds a 5M-key ring per subtest and runs a full warmup DoBatch inside each setup to derive a timeout. Hoist ring/keys construction outside the subtest loop and measure the warmup once.
• TestSpreadMinimizingTokenGenerator_GenerateAllTokensIdempotent: idempotence is a per-(instance, zone) property; 128 instances are overkill. Reduce to 16.
• TestSpreadMinimizingTokenGenerator_CheckTokenUniqueness: uniqueness is a generator invariant; reduce instanceID 10000 → 1000 (still 1.5M tokens covered).
• TestCheckingOfKeyOwnership: drop the two largest user-token sizes (50k, 100k); the property holds at every scale.

Measured locally with go test -race -count 1 ./ring/: 492s → 285s (−42%). Expected CI impact: group 0 drops from ~13min to ~7–8min.

Which issue(s) this PR fixes:

N/A

Checklist

• Tests updated

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Note

Low Risk
Low risk: changes are confined to tests and primarily reduce workloads and redundant setup; main risk is reduced coverage for very-large-scale scenarios.

Overview
Speeds up ring package unit tests by removing redundant heavy setup and shrinking pathological test inputs (especially under -race).

TestDoBatchWithOptionsContextCancellation now builds the ring/5M keys once and performs a single warmup DoBatch to derive timeouts for subtests, instead of re-creating the ring/keys and re-measuring per subtest.

Reduces runtime in token-generator/ownership tests by lowering iteration ranges: fewer instance IDs in SpreadMinimizingTokenGenerator idempotence/uniqueness tests and fewer large token-count cases in TestCheckingOfKeyOwnership.

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