Completed the needed context to successfully test commands
bug #4582 Completed the needed context to successfully test commands with Helpers (peterrehm)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4582).

Discussion
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Completed the needed context to successfully test commands with Helpers

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4581

As mentioned in symfony/symfony#12798 the explanation about the initialization is not clear.
Therefore I have added the needed context and a reference to the related cookbook article.

Commits
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a94bd71 Completed the needed context to successfully test commands