Remove test skipping on connection error

[philippgille]

In the past, probably before we had the mage test all that automatically spins up Docker containers for the different store implementation tests, it was useful to do connection tests and skip tests when no connection could be established, to not fail an entire test run, but instead allow to check in the output log for any unexpected skips.

The Docker container start has been very reliable though, while OTOH I've been less often checking for skipped tests, and it's also cumbersome to check with the long test output (and not using tparse yet).

=> This PR removes the connection check and test skipping from almost all store implementation tests.

The exceptions:

• tablestorage and tablestore: Their test containers don't yet work correctly. So far they need to be tested locally, with actual credentials to the production cloud services. We should revisit their documentation, because since I last checked they might have improved/fixed their Docker images or improved their documentation on how to make them work.
• MySQL: For some reason the tests work fine locally (always) but never work in CI. Haven't found out why yet. As an improvement over the previous state we replace the connection test with a check for the test environment and only skip the test in CI, but not locally.

We leave the checkConnection functions in the test files even if they're not called, because they're very handy as soon as connection issues pop up, to find out if it's from the gokv client specific code or the connection in general.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6bcd482) 63.64% compared to head (7742dbb) 63.64%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #173   +/-   ##
=======================================
  Coverage   63.64%   63.64%           
=======================================
  Files          25       25           
  Lines        2107     2107           
=======================================
  Hits         1341     1341           
  Misses        651      651           
  Partials      115      115           

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