minor #11582 Update testing.rst (iiirxs)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #11582).

Discussion
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Update testing.rst

getAttribute function doesn't exist in Crawler class: use attr() instead: Error: Call to undefined method Symfony\Component\DomCrawler\Crawler::getAttribute()

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Commits
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0c05c83 Update testing.rst

Author: javiereguiluz

testing.rst

@@ -626,7 +626,7 @@ narrow down your node selection by chaining the method calls::
     $crawler
         ->filter('h1')
         ->reduce(function ($node, $i) {
-            if (!$node->getAttribute('class')) {
+            if (!$node->attr('class')) {
                 return false;
             }
         })