fix: make tests skippable with an environment variable

[carolynsun-google]

Public zone requests to Cloud DNS are blocked when enforcing VPCSC. Make tests that involve public zone requests skippable.

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[codecov]

Codecov Report

Merging #357 (f044856) into master (d748c6f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #357   +/-   ##
=========================================
  Coverage     77.26%   77.26%           
  Complexity      178      178           
=========================================
  Files            15       15           
  Lines          1192     1192           
  Branches        105      105           
=========================================
  Hits            921      921           
  Misses          218      218           
  Partials         53       53           

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