test: improve GatewayWithAttachedRoutes conformance test

[mlavacca]

What type of PR is this?

/kind test
/area conformance

What this PR does / why we need it:

The GatewayWithAttachedRoutes conformance test has been improved such that when a route references a non-existing listener in a Gateway through the SectionName, the number of AttachedRoutes does not get incremented.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mlavacca

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• conformance/OWNERS [mlavacca]

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[k8s-ci-robot]

@mlavacca: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-crds-validation-4	5579e38	link	true	/test pull-gateway-api-crds-validation-4

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[k8s-triage-robot]

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/lifecycle rotten