conformance: initial draft for CORS filter tests

[snorwin]

What type of PR is this?

/kind test
/area conformance-test

What this PR does / why we need it:
During the implementation of the new CORS HTTPRoute filter, I created additional tests that can serve as a foundation for developing conformance tests for the CORS feature (#1767).

The proposed tests focus on:

• CORS preflight requests
• GET requests with Origin header set

Open Points

• Test case with * as an allowed origin (currently not possible due to CORS documentation and validation are not consistent #3648)
• Handling multiple valid status codes in the response (currently, only status code 200 is considered valid in this tests).

Feedback is welcome to enhance the tests and address the open items effectively.

Does this PR introduce a user-facing change?:

TODO

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @snorwin. Thanks for your PR.

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[howardjohn]

Isn't ExpectedRequest the request that hits the backend? Wouldn't this not hit the backend at all?

[snorwin]

Yes, exactly. However ExpectedRequest: nil implicitly sets the actual request as expected request, which isn't correct for this case.

Also. when I tried to set the expected request to be empty with ExpectedRequest: &http.ExpectedRequest{}, it still didn't work. This is because the method of the captured request is set to the method of the actual request, even if there there is no captured request at all (https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/utils/roundtripper/roundtripper.go#L252).

If you have a more effective approach to assert that no request was sent to the backend, please let me know.

[snorwin]

Closing in the favor of #3739.