use resty-cli for running lua unit tests

[ElvinEfendi]

What this PR does / why we need it:
This way we will run Lua tests inside a headless Nginx process. That means we won't have to mock most of the dependencies and the test environment will be very close to production environment.
resty-cli runs the Lua code in timer phase so only the APIs that aren't available in timer phase won't be available.

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:

[ElvinEfendi]

/assign @aledbf
/assign @antoineco

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [ElvinEfendi,aledbf]

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[codecov-io]

Codecov Report

Merging #2829 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2829   +/-   ##
=======================================
  Coverage   47.34%   47.34%           
=======================================
  Files          75       75           
  Lines        5413     5413           
=======================================
  Hits         2563     2563           
  Misses       2525     2525           
  Partials      325      325

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