Add missing exclusions from phpunit.xml.dist

Author: nicolas-grekas

phpunit.xml.dist

@@ -9,6 +9,7 @@
     <php>
         <ini name="error_reporting" value="-1" />
     </php>
+
     <testsuites>
         <testsuite name="Symfony OptionsResolver Component Test Suite">
             <directory>./Tests/</directory>
@@ -21,6 +22,7 @@
             <exclude>
                 <directory>./Resources</directory>
                 <directory>./Tests</directory>
+                <directory>./vendor</directory>
             </exclude>
         </whitelist>
     </filter>