improve annotations/default_backend e2e test

[ElvinEfendi]

What this PR does / why we need it:
Instead of asserting on Nginx config we now test by sending an actual request and asserting on expected response. This also makes sure the right headers are set for custom default backend.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:
This PR is going to fix CI for #2794.

[ElvinEfendi]

/assign @aledbf
/assign @antoineco

[codecov-io]

Codecov Report

Merging #2853 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2853      +/-   ##
==========================================
- Coverage   47.39%   47.37%   -0.02%     
==========================================
  Files          75       75              
  Lines        5433     5433              
==========================================
- Hits         2575     2574       -1     
- Misses       2530     2532       +2     
+ Partials      328      327       -1

Impacted Files	Coverage Δ
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

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[antoineco]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: antoineco, ElvinEfendi

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