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

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: snorwin
Once this PR has been reviewed and has the lgtm label, please assign robscott for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[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