Add unit tests to configuration_test.lua that cover Backends configuration

[fmejia97]

What this PR does / why we need it:
This PR adds additional unit tests to configuration lua module because there are no tests covering Backends configuration.

[codecov-io]

Codecov Report

Merging #2946 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2946   +/-   ##
=======================================
  Coverage   47.57%   47.57%           
=======================================
  Files          77       77           
  Lines        5532     5532           
=======================================
  Hits         2632     2632           
  Misses       2560     2560           
  Partials      340      340

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, fmejia97

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