Added sample "jest" tests for counter reducer #509

[rosieks]

This PR fills the gap between Angular and React-Redux template by adding test library and sample tests to the second one. #509

[dnfclas]

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[SteveSandersonMS]

Thanks for the suggestion!

These jest tests are not really equivalent to the karma tests in the Angular project, because the karma ones are testing at a UI level by automating the browser, whereas these jest tests are purely unit tests that don't describe the UI at all.

If we were to add sample tests to the Redux project, I think we'd want to add higher-level UI tests for equivalence. But that would still only be if those tests and its setup/config were all simple enough to be easily understood by a total newcomer.

Is that OK? Do you mind if I close this PR?

[rosieks]

That make sense.

[SteveSandersonMS]

Thanks!