minor symfony#8983 cross-reference console command testing from testing guide (dbu)

This PR was merged into the 4.0 branch.

Discussion
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cross-reference console command testing from testing guide

this feature is "hidden" in the console documentation. it belongs there, but we should find it from the testing guide as well.

Commits
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d0dfade cross-reference console testing to help users find this feature

Author: javiereguiluz

testing.rst

@@ -919,6 +919,7 @@ Learn more
 
     testing/*
 
+* :ref:`Testing a console command <console-testing-commands>`
 * :doc:`The chapter about tests in the Symfony Framework Best Practices </best_practices/tests>`
 * :doc:`/components/dom_crawler`
 * :doc:`/components/css_selector`