Extend DomainMapping conformance test to check DomainClaims

[julz]

Part of #9713.

Extends the DomainMapping conformance test to validate that colliding DomainMappings work correctly.

/assign @mattmoor

[codecov]

Codecov Report

Merging #10093 (4339aac) into master (db4879e) will decrease coverage by 0.07%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master   #10093      +/-   ##
==========================================
- Coverage   88.01%   87.94%   -0.08%     
==========================================
  Files         185      185              
  Lines        8686     8708      +22     
==========================================
+ Hits         7645     7658      +13     
- Misses        793      798       +5     
- Partials      248      252       +4     

Impacted Files	Coverage Δ
pkg/reconciler/revision/revision.go	87.09% <66.66%> (-2.38%)	⬇️
pkg/reconciler/route/traffic/rollout.go	100.00% <100.00%> (ø)
pkg/reconciler/configuration/configuration.go	85.93% <0.00%> (-2.35%)	⬇️
pkg/reconciler/autoscaling/kpa/scaler.go	88.57% <0.00%> (-1.43%)	⬇️
pkg/activator/net/revision_backends.go	90.49% <0.00%> (-0.91%)	⬇️

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[mattmoor]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz, mattmoor

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