CPU Scaler: Make CPU scaler test wait for metrics window

[jkyros]

The CPU scaler tests assume the default metrics-server window of 30s, so those testing on platforms with larger metrics windows (e.g. platforms using prometheus-adapter, which has a 5m default window) will fail the CPU scaler test because the metrics won't be ready by the time the test starts.

This:

• Adds a helper that waits for either the metrics to show up in the HPA, or for some amount of time to pass, whichever happens first
• Uses said helper to ensure that the metrics are ready before the CPU test starts testing scaling

Output looks like this when it runs:

helper.go:441: Waiting for deployment replicas to hit target. Deployment - cpu-test-deployment, Current  - 1, Target - 1
helper.go:414: Waiting up to 10m0s for HPA to populate metrics - 0s so far
helper.go:414: Waiting up to 10m0s for HPA to populate metrics - 5s so far
...
helper.go:414: Waiting up to 10m0s for HPA to populate metrics - 3m35s so far
helper.go:414: Waiting up to 10m0s for HPA to populate metrics - 3m40s so far
helper.go:423: HPA has metrics after 3m40.333531912s: [
      {
          "type": "Resource",
          "resource": {
              "name": "cpu",
              "current": {
                  "averageValue": "1m",
                  "averageUtilization": 0
              }
          }
      }
  ]
cpu_test.go:210: --- testing scale out ---

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

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[JorTurFer]

/run-e2e cpu
Update: You can check the progress here