Add missing integration test configuration

[chingor13]

• Adds Kokoro config for integration tests for all clients regardless of whether or not there are any integration tests written yet
• Executes smoke tests along with the integration tests

[codecov]

Codecov Report

Merging #5623 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #5623   +/-   ##
=========================================
  Coverage     46.71%   46.71%           
  Complexity    24627    24627           
=========================================
  Files          2351     2351           
  Lines        256125   256125           
  Branches      29325    29325           
=========================================
  Hits         119648   119648           
  Misses       127559   127559           
  Partials       8918     8918

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