conformance: Add all zero weight backend tests

[optix2000]

What type of PR is this?
/kind test
/area conformance-test

What this PR does / why we need it:
Tests alternate case where all backends have 0 weight.
While not explicitly defined, it's implied that these should also return 500/Unavailable.

Discussed originally here: #4750 (comment)

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: optix2000
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[k8s-ci-robot]

Hi @optix2000. 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.

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[k8s-triage-robot]

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