test(proxy): fix flaky TestShadow timing and counter assertions

[shardool-patil]

Summary

Fixes non-deterministic failures in TestShadow caused by rigid timing thresholds & Vegeta request count variance under CI load

Changes

1. FIFO Shedding Assertions: Replaced the rigid statusFifoFull < 2 check with cumulative capacity validation (statusFifoFull + statusFifoTimeout >= 10 and statusFifoTimeout >= 1) to account for sub-millisecond inter-arrival timer drift and wait timeouts under heavy CPU load
2. Request Count Tolerance: Decoupled success validation from strict nominal target equality ($N = 100$), comparing statusOK and counterMain against completed requests (reqCount with a tolerance bound of reqCount >= N-5) to prevent false-positive failures due to attack timer truncation

Related Issues

• Fixes Flaky TestShadow #2683

[szuecs]

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[a4180p]

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