add link to form testing chapter in test section
minor #4190 add link to form testing chapter in test section (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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add link to form testing chapter in test section

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

The confusion in symfony/symfony#11774 made me think that we could improve the links between the testing and the form section when it comes to testing forms.

Commits
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a2dbd36 add link to form testing chapter in test section