Switch to yaml files for test data

[levikobi]

What would you like to be added: Currently, we have input/output test data defined in the tests themselves, making them hard to read. I suggest switching to YAML files instead, similar to what we already have in the Istio package.

Why this is needed: To improve the maintenance and extensibility of our tests.

[levikobi]

/help

[k8s-ci-robot]

@levikobi: This request has been marked as needing help from a contributor.

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