conformance: normative test for backend client certificate in Gateway

[snorwin]

What type of PR is this?

/kind test
/area conformance-test

What this PR does / why we need it:
Adds a normative conformance test for .spec.tls.backend.clientCertificateRef. The test verifies that the configured client certificate is presented when connecting to a backend over TLS.

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

[snorwin]

/cc @rikatz @kl52752

No rush on this, just whenever you have a moment. I was working on the feature in our implementation and thought it might be a good idea to propose a conformance test early.

[rikatz]

/assign
Will take a look next week :D happy weekend!

[snorwin]

/test all

[kl52752]

thanks for PR, LGTM!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kl52752, snorwin
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[snorwin]

/assign @robscott