Test suite fails if .enthought/unittest/preferences.ini already exists

[mdickinson]

Steps to reproduce (Linux / macOS):

• create a .enthought/unittest/preferences.ini file with the following contents:

$ cat ~/.enthought/unittest/preferences.ini
[app]
server_url = https://my.server.com

• run the test suite

Then (for me), the test suite fails with the following error:

======================================================================
FAIL: test_node_names (apptools.preferences.tests.test_scoped_preferences.ScopedPreferencesTestCase.test_node_names)
node names
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/mdickinson/Enthought/ETS/apptools/apptools/preferences/tests/test_preferences.py", line 242, in test_node_names
    self.assertEqual([], p.node_names())
AssertionError: Lists differ: [] != ['app']

Second list contains 1 additional elements.
First extra element 0:
'app'

- []
+ ['app']