Use len of set to check for duplicates in list of packages.

Resolves: #154.

Checking for equality of the two lists was a mistake, since
the order of the list generated from the set is undefined.

tests/devices_test/device_packages_test.py

@@ -20,7 +20,7 @@ def testPackages(self):
         packages = luks.packages
 
         # no duplicates in list of packages
-        self.assertListEqual(packages, list(set(packages)))
+        self.assertEqual(len(packages), len(set(packages)))
 
         # several packages that ought to be included are
         for package in dev1.packages + dev2.packages + dev.packages: