fix bug with lua e2e test suite

[ElvinEfendi]

What this PR does / why we need it:

• fix the regression introduced at https://github.com/kubernetes/ingress-nginx/pull/2804/files#diff-8700c46a8379c603b5e41ccbaff6b71eR59
• extract sleep time into a constant and reduce the time

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]

As you can see I wasn't asserting on err here and the check was timing out (because the condition never matches, executing dns.GetSystemNameServers() returns resolvers for the current host not for the running pod) silently after 5 minutes, and this was happening before every test. Therefore e2e test runtime doubled.

[ElvinEfendi]

/assign @aledbf
/assign @antoineco

[codecov-io]

Codecov Report

Merging #2867 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2867   +/-   ##
=======================================
  Coverage   47.71%   47.71%           
=======================================
  Files          75       75           
  Lines        5434     5434           
=======================================
  Hits         2593     2593           
  Misses       2510     2510           
  Partials      331      331

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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