Sample functional test is flaky

[kate-osborn]

The sample functional test periodically fails in the pipeline:

Basic test example sends traffic [functional]
/home/runner/work/nginx-gateway-fabric/nginx-gateway-fabric/tests/suite/sample_test.go:40
  [FAILED] in [It] - /home/runner/work/nginx-gateway-fabric/nginx-gateway-fabric/tests/suite/sample_test.go:47 @ 05/28/24 14:34:12.349
• [FAILED] [7.077 seconds]
Basic test example [It] sends traffic [functional]
/home/runner/work/nginx-gateway-fabric/nginx-gateway-fabric/tests/suite/sample_test.go:40

  [FAILED] Expected
      <int>: 502
  to equal
      <int>: 200
  In [It] at: /home/runner/work/nginx-gateway-fabric/nginx-gateway-fabric/tests/suite/sample_test.go:47 @ 05/28/24 14:34:12.349

Pipeline run: https://github.com/nginxinc/nginx-gateway-fabric/actions/runs/9271134013/job/25506194841?pr=2013

[sjberman]

This will be fixed in #2004, I've added retries.