minor #1397 Bootstrap composer autoload in PHPUnit (MatthieuMota)

This PR was submitted for the master branch but it was merged into the 1.3 branch instead (closes #1397).

Discussion
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Bootstrap composer autoload in PHPUnit

Avoid to require autoload in test class.

Commits
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cd8ee1a8 Bootstrap composer autoload in PHPUnit

doc/testing.rst

@@ -182,7 +182,8 @@ look like this:
 .. code-block:: xml
 
     <?xml version="1.0" encoding="UTF-8"?>
-    <phpunit backupGlobals="false"
+    <phpunit bootstrap="./vendor/autoload.php"
+             backupGlobals="false"
              backupStaticAttributes="false"
              colors="true"
              convertErrorsToExceptions="true"