Support failure if integration tests cannot run.

[smoser]

Previously intergration tests would just Skip() if they did not have the necessary tools to run. That could lead to a situation where we don't realize the tests are not running and happily trust c-i that was not running integration tests.

Here, we let the behavior be controlled by DISKO_INTEGRATION environment variable.

• allow-skip (default): act as before (run if possible, or skip)
• skip: always skip.
• run: raise errors and fail if the conditions are not met.

Also, we set DISKO_INTEGRATION=run in c-i, so failures will fail.

[codecov[bot]]

Codecov Report

Merging #88 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   66.23%   66.23%           
=======================================
  Files          16       16           
  Lines        1937     1937           
=======================================
  Hits         1283     1283           
  Misses        487      487           
  Partials      167      167           

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