test: dns-liveness against more than one URL

[jackfrancis]

Reason for Change:

Validate DNS against more than one external URL to more reliably determine that our external DNS resolution is really broken.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[acs-bot]

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[codecov]

Codecov Report

Merging #2016 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2016   +/-   ##
=======================================
  Coverage   76.73%   76.73%           
=======================================
  Files         135      135           
  Lines       20574    20574           
=======================================
  Hits        15787    15787           
  Misses       3871     3871           
  Partials      916      916