selftests/unit/test_utils_process.py: syntax fix for skipUnless

Commit 33517beb has a syntax
failure because reason is a mandatory argument.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/unit/test_utils_process.py

@@ -172,7 +172,8 @@ class TestProcessRun(unittest.TestCase):
         p = process.run(cmd='ls -l', ignore_status=True)
         self.assertEqual(p.command, expected_command)
 
-    @unittest.skipUnless(os.path.exists('/bin/sudo'))
+    @unittest.skipUnless(os.path.exists('/bin/sudo'),
+                         "/bin/sudo not available")
     @mock.patch.object(path, 'find_command',
                        mock.Mock(return_value='/bin/sudo'))
     @mock.patch.object(os, 'getuid', mock.Mock(return_value=1000))