fix some deprecation warnings in tests

Calling HasTraits(attr=value) doesn't work for non-traits

1. remove one redundant assignment for a non-trait
2. make one attribute a trait

jupyterhub/tests/conftest.py

@@ -76,7 +76,7 @@ def app(request, io_loop, ssl_tmpdir):
     """Mock a jupyterhub app for testing"""
     mocked_app = None
     ssl_enabled = getattr(request.module, "ssl_enabled", False)
-    kwargs = dict(log_level=logging.DEBUG)
+    kwargs = dict()
     if ssl_enabled:
         kwargs.update(
             dict(

jupyterhub/tests/mocking.py

@@ -40,7 +40,7 @@ from tornado import gen
 from tornado.concurrent import Future
 from tornado.ioloop import IOLoop
 
-from traitlets import Bool, default
+from traitlets import Bool, Dict, default
 
 from ..app import JupyterHub
 from ..auth import PAMAuthenticator
@@ -219,12 +219,15 @@ class MockPAMAuthenticator(PAMAuthenticator):
 class MockHub(JupyterHub):
     """Hub with various mock bits"""
 
+    # disable some inherited traits with hardcoded values
     db_file = None
     last_activity_interval = 2
     log_datefmt = '%M:%S'
-    external_certs = None
     log_level = 10
 
+    # MockHub additional traits
+    external_certs = Dict()
+
     def __init__(self, *args, **kwargs):
         if 'internal_certs_location' in kwargs:
             cert_location = kwargs['internal_certs_location']