Improve (integration) testing

[fabriziosestito]

Is your feature request related to a problem?

At the moment, to test evalution/runtime related feature we need to run e2e tests in policy-server.
We could rewrite (part of) these tests as rust integration tests.
See: https://github.com/kubewarden/policy-server/tree/main/e2e-tests
Also the policy server itself doesnt' have many tests regarding rest API handlers, so it might be worth it adding those.

Solution you'd like

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Alternatives you've considered

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Anything else?

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