cleanup: rename integration test utilities to remove _test suffix

[LukeAVanDrie]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This PR renames utility and harness files within the EPP integration test suite to align with project conventions and fix linting issues.

Previously, harness.go and util.go carried the _test.go suffix. While this correctly excluded them from the production binary via build constraints, it violated the convention that _test.go files should primarily contain test functions.

Changes:

1. Rename: harness_test.go -> harness.go, util_test.go -> util.go.
2. Globals Refactor: Moved global test variables (k8sClient, testEnv, etc.) from hermetic_test.go to harness.go. This ensures that the newly renamed (non-test) files can compile and access these shared resources without import cycles or scope visibility errors.
3. Utility Consolidation: Moved the cleanMetric helper to util.go.

Which issue(s) this PR fixes:
Follow-up to feedback from PR #2022.

Does this PR introduce a user-facing change?:

NONE

[netlify]

✅ Deploy Preview for gateway-api-inference-extension ready!

Name	Link
🔨 Latest commit	f78d101
🔍 Latest deploy log	https://app.netlify.com/projects/gateway-api-inference-extension/deploys/695d8b7c6f6d6f0008a37dee
😎 Deploy Preview	https://deploy-preview-2084--gateway-api-inference-extension.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[k8s-ci-robot]

Hi @LukeAVanDrie. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[LukeAVanDrie]

/assign @ahg-g

I didn't create an issue for this one, but this is a followup from our discussion in #2022.

[ahg-g]

/ok-to-test

[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, LukeAVanDrie

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [ahg-g]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment