[8.0] printer_tray: Add unit tests (#64)

* [ADD] printer_tray: Add unit tests

* [FIX] Fixed lint errors

printer_tray/printer.py

@@ -18,12 +18,20 @@
 #    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 ##############################################################################
-import cups
+
 import errno
+import logging
 import os
 
 from openerp import models, fields, api
 
+_logger = logging.getLogger(__name__)
+
+try:
+    import cups
+except ImportError:
+    _logger.debug('Cannot `import cups`.')
+
 
 class Printer(models.Model):
     _inherit = 'printing.printer'