admission control: Use less iterations in integration test

[tonya11en]

The admission control integration test occasionally times out when running through CI with a TSAN build (#11758). To address this, the admission control integration test now performs less iterations.

Local testing does not exhibit the timeouts observed in CI after ~1000 runs. Prior to this patch it was easily reproduced.

//test/extensions/filters/http/admission_control:admission_control_integration_test PASSED in 99.0s
  Stats over 1000 runs: max = 99.0s, min = 96.1s, avg = 97.7s, dev = 0.4s

Risk Level: Negligible
Testing: 1000 TSAN runs of integration test
Docs Changes: n/a
Release Notes: n/a

[mattklein123]

Thanks!

[asraa]

@tonya11en Could you try merging master again? I'm not sure what the reason for the circle ci tests not passing, but that has fixed the issue before @lizan