Fix the test for casting FlattenException on Windows

The assertion should avoid forcing to show path separators, as done in
other tests of the caster, as path separators are different on Windows.

Tests/Caster/ExceptionCasterTest.php

@@ -375,7 +375,7 @@ public function testFlattenException()
     -statusCode: 500
     -statusText: "Internal Server Error"
     -headers: []
-    -file: "%s/src/Symfony/Component/VarDumper/Tests/Caster/ExceptionCasterTest.php"
+    -file: "%sExceptionCasterTest.php"
     -line: %d
     -asString: null
   }