Add unit test cases for balancer lua module

[s-shirayama]

What this PR does / why we need it:
This PR adds unit test cases for balancer.lua's route_to_alternative_balancer() function.
Since I'm planning to fix #4028 and add more test cases for the change, I added test cases for existing logic of target function.

Which issue this PR fixes *(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) Related with #4028 (but not fix it)

Special notes for your reviewer:
N/A

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @s-shirayama. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[aledbf]

/ok-to-test

[s-shirayama]

/retest

[s-shirayama]

/test pull-ingress-nginx-test

[codecov-io]

Codecov Report

Merging #4187 into master will increase coverage by 0.93%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4187      +/-   ##
=========================================
+ Coverage   57.66%   58.6%   +0.93%     
=========================================
  Files          87      87              
  Lines        6463    6950     +487     
=========================================
+ Hits         3727    4073     +346     
- Misses       2306    2409     +103     
- Partials      430     468      +38

Impacted Files	Coverage Δ
internal/ingress/metric/collectors/process.go	88.29% <0%> (-2.13%)	⬇️
cmd/nginx/flags.go	86.66% <0%> (-1.78%)	⬇️
internal/ingress/controller/controller.go	47.84% <0%> (+1.33%)	⬆️
internal/ingress/controller/template/template.go	87.35% <0%> (+3.11%)	⬆️

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[s-shirayama]

pull-ingress-nginx-test job seems unstable..

1st trial : failed
2nd trial : failed
3rd trial : passed

[s-shirayama]

Hi, @aledbf
What are the remaining works for this PR to be merged ?

[aledbf]

@s-shirayama review from @ElvinEfendi

[ElvinEfendi]

@s-shirayama I love these kind of PRs - thanks!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ElvinEfendi, s-shirayama

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• OWNERS [ElvinEfendi]

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