Add test utility to return an id_list

Makes creating an id_list for testing from a list test instances
easier.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/common/tests/utils.py

@@ -1,6 +1,10 @@
+from __future__ import absolute_import, unicode_literals
+
 from contextlib import contextmanager
 import sys
 
+from django.utils.encoding import force_text
+
 
 class NullFile(object):
     def write(self, string):
@@ -13,3 +17,9 @@ def mute_stdout():
     sys.stdout = NullFile()
     yield
     sys.stdout = stdout_old
+
+
+def as_id_list(items):
+    return ','.join(
+        [force_text(item.pk) for item in items]
+    )