Use networking helpers in AutoTLS tests instead of copying them

[markusthoemmes]

Proposed Changes

As per title, these seem to be straight up duplicates.

Release Note

NONE

/assign @nak3 @tcnghia

[knative-prow-robot]

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This pull-request has been approved by: markusthoemmes

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[codecov]

Codecov Report

Merging #10052 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10052      +/-   ##
==========================================
+ Coverage   87.98%   88.01%   +0.02%     
==========================================
  Files         184      185       +1     
  Lines        8663     8683      +20     
==========================================
+ Hits         7622     7642      +20     
+ Misses        794      793       -1     
- Partials      247      248       +1     

Impacted Files	Coverage Δ
pkg/reconciler/domainmapping/reconciler.go	85.18% <0.00%> (-0.53%)	⬇️
pkg/reconciler/route/domains/domains.go	100.00% <0.00%> (ø)
.../reconciler/domainmapping/resources/domainclaim.go	100.00% <0.00%> (ø)
pkg/activator/net/revision_backends.go	91.40% <0.00%> (+0.90%)	⬆️

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[nak3]

/lgtm