Unit Tests: Implement `addWarning()` method in SpeedTrapListener.

The method was introduced in PHPUnit 5.1.0, released today.

Fixes #34846.

git-svn-id: https://develop.svn.wordpress.org/trunk@35767 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/speed-trap-listener.php

@@ -59,6 +59,18 @@ class SpeedTrapListener implements PHPUnit_Framework_TestListener
     {
     }
 
+    /**
+     * A warning occurred.
+     *
+     * @param PHPUnit_Framework_Test    $test
+     * @param PHPUnit_Framework_Warning $e
+     * @param float                     $time
+     * @since Method available since Release 5.1.0
+     */
+    public function addWarning(PHPUnit_Framework_Test $test, PHPUnit_Framework_Warning $e, $time)
+    {
+    }
+
     /**
      * A failure occurred.
      *