minor #21525 Document setting TEST_GENERATE_FIXTURES=1 to update snapshots (GromNaN)

This PR was merged into the 7.4 branch.

Discussion
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 Document setting `TEST_GENERATE_FIXTURES=1` to update snapshots

Document this PR: symfony/symfony#62121

Commits
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cd605ce Document TEST_GENERATE_FIXTURES for snapshot tests

Author: javiereguiluz

contributing/code/tests.rst

@@ -64,6 +64,18 @@ what's going on and if the tests are broken because of the new code.
     On Windows, install the `Cmder`_, `ConEmu`_, `ANSICON`_ or `Mintty`_ free applications
     to see colored test results.
 
+Testing Generated Code
+----------------------
+
+Some tests generate code on the fly and check that it matches expected stored in
+in a file. To regenerate those files, you can use the following approach, run
+the tests with the environment variable ``TEST_GENERATE_FIXTURES`` set to ``1``:
+
+.. code-block:: terminal
+
+    $ TEST_GENERATE_FIXTURES=1 php ./phpunit src/Symfony/Component/Config/Tests/Builder/GeneratedConfigTest.php
+
+
 .. _`install Composer`: https://getcomposer.org/download/
 .. _Cmder: https://cmder.app/
 .. _ConEmu: https://conemu.github.io/