[Debug] Fix undefined variable $lightTrace

* 3.3:
  [Bridge/PhpUnit] Fix compat with phpunit 4.8 & bridge <=3.3.13
  Remove function_exists(__phpunit_run_isolated_test) checks

ErrorHandler.php

@@ -417,6 +417,7 @@ class ErrorHandler
                 $errorAsException = self::$silencedErrorCache[$id][$message];
                 ++$errorAsException->count;
             } else {
+                $lightTrace = array();
                 $errorAsException = null;
             }
 

Tests/Fixtures/Throwing.php

@@ -1,5 +1,3 @@
 <?php
 
-if (!function_exists('__phpunit_run_isolated_test')) {
-    throw new \Exception('boo');
-}
+throw new \Exception('boo');