Increase Subscription validation test coverage

[aliok]

Part of #4931

Proposed Changes

• 🧹 Increase test coverage for Subscription validation

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Docs

[knative-prow-robot]

@aliok: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-knative-eventing-go-coverage	18b9424	link	/test pull-knative-eventing-go-coverage

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[codecov]

Codecov Report

Merging #5305 (18b9424) into main (83a1713) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5305      +/-   ##
==========================================
+ Coverage   83.64%   83.67%   +0.02%     
==========================================
  Files         243      243              
  Lines        6896     6896              
==========================================
+ Hits         5768     5770       +2     
+ Misses        785      784       -1     
+ Partials      343      342       -1     

Impacted Files	Coverage Δ
pkg/apis/messaging/v1/subscription_validation.go	85.00% <0.00%> (+5.00%)	⬆️

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[pierDipi]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pierDipi

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