minor #13983 Change unit test definition (l-vo)

This PR was submitted for the 3.4 branch but it was merged into the 4.4 branch instead.

Discussion
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Change unit test definition

I'm not sure about the unit test definition. Considering unit tests as testing against a single class often leads to over-mocking. What about talking of single behaviour instead ?

Commits
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3324630 Change unit test definition

Author: javiereguiluz

testing.rst

@@ -57,8 +57,8 @@ Symfony application.
 Unit Tests
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-A unit test is a test against a single PHP class, also called a *unit*. If you
-want to test the overall behavior of your application, see the section about
+A unit test is a test against a single behavior, also called a *unit*. If you
+want to test the overall way of your application, see the section about
 :ref:`Functional Tests <functional-tests>`.
 
 Writing Symfony unit tests is no different from writing standard PHPUnit