Tests freeze if fail is called with an Exception object

If you create a test case and the assert use an Exception (or child) object, it freeze the code trying to allocate all the memory available for PHP.

Debugging the code I saw the problem is caused by the `print_r` in `PHPUnit_Util_Type::recursiveExport`.

It seems a PHP bug, but maybe PHPUnit can use a different approach for Exception classes to avoid it. The easiest way was removing the trace, once it is not very important on tests dumps.

Thanks for @cjsaylor in help figure this out.