fix(admission): use Ignore failure policy in tests (same as charts)

[czeslavo]

What this PR does / why we need it:

Uses the same failure policy (Ignore) in the test admission webhook configuration as the one used in charts and deploy-admission-controller.sh script.

The policy tells Kubernetes what to do in case status 500 is returned from the webhook:

• Ignore - accept the object.
• Fail - do not accept the object.

Given that we're using Ignore in production, we should use the same in tests. Without that, we may have test cases where we expect an object to be rejected, but we returned status 500, which in production would still accept the object.

What validations will this bring back to life for installations using default failurePolicy:

• HTTPRoute:
  • queryparam matching is supported with expression router only
  • group is not a supported group for httproute backendRefs, only core is supported
  • kind is not a supported kind for httproute backendRefs, only Service is supported
  • no parentRef matched gateway
  • sectionname referenced listener was not found on gateway
  • no listeners could be found for gateway
  • HTTPRoute not supported by listener
  • HTTPRoute failed schema validation (route could not be generated in parser)
  • referenced gateway not found
  • referenced gatewayclass not found
• KongConsumer:
  • failed to fetch managed KongConsumers from cache
  • consumer referenced non-existent credentials secret
  • consumer credential failed validation

Which issue this PR fixes:

Related issue: #4680

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (6fb8469) 73.3% compared to head (3e3c7cb) 75.6%.
Report is 3 commits behind head on main.

❗ Current head 3e3c7cb differs from pull request most recent head ac4860d. Consider uploading reports for the commit ac4860d to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5063     +/-   ##
=======================================
+ Coverage   73.3%   75.6%   +2.2%     
=======================================
  Files        167     167             
  Lines      18909   18911      +2     
=======================================
+ Hits       13867   14299    +432     
+ Misses      4218    3788    -430     
  Partials     824     824             

Files	Coverage Δ
internal/admission/handler.go	60.5% <100.0%> (-0.6%)	⬇️
internal/admission/validation/gateway/httproute.go	90.0% <81.8%> (+<0.1%)	⬆️
internal/admission/validator.go	72.7% <42.8%> (-0.7%)	⬇️

... and 25 files with indirect coverage changes

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