simplify BackendTLSPolicy test infrastructure and remove unnecessary code

[snorwin]

What type of PR is this?

/kind test
/area conformance-machinery

What this PR does / why we need it:
This PR simplifies the test infrastructure for BackendTLSPolicy by reusing the existing backend-tls Deployment instead of creating a separate backend. In addition, it fixes the CA certificate creation process as a CA certificates typically do not contain hostnames and following best practices, the CA certificate private key is now omitted from the ConfigMap.

The refactoring was validated by re-running the BackendTLSPolicy tests against Envoy Gateway and Airlock Microgateway, all of which passed successfully.

Which issue(s) this PR fixes:

Fixes #3934

Does this PR introduce a user-facing change?:

NONE

[snorwin]

/cc @candita @kl52752

[snorwin]

In case that #3983 is merged before this PR, the changes here will also need to be applied to the BackendTLSPolicySANValidation tests.

[kl52752]

Thanks for cleaning this up! One question impacting SAN tests that are in review

[snorwin]

@shaneutt, it would be great if we could get this merged before the code freeze.

[kl52752]

Thanks for the PR, looks good now :)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

/lgtm

[snorwin]

In the community meeting, we agreed to wait on merging this until #3983 is merged.
/hold
/cc @kl52752

[rikatz]

why remove the test? we still have the generateCACert helper function right? I mean, was this test failing or is this test not being useful?

[snorwin]

It was only for testing that the CA certificate contained the specified hostnames, which does not make sense.