lib/test_printf: Add tests for %pfw printk modifier

Add a test for the %pfw printk modifier using software nodes.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>