Use a temporary file for the application_preferences_filename (#338)

Closes #337 

This PR ensures that tests of `ScopedPreferences` use a temporary file
for the application preferences scope.

apptools/preferences/tests/test_preferences_helper.py

@@ -411,7 +411,9 @@ class AcmeUIPreferencesHelper(PreferencesHelper):
     def test_scoped_preferences(self):
         """ scoped preferences """
 
-        p = set_default_preferences(ScopedPreferences())
+        p = set_default_preferences(
+            ScopedPreferences(application_preferences_filename=self.tmpfile)
+        )
 
         # Set a preference value in the default scope.
         p.set("default/acme.ui.bgcolor", "blue")

apptools/preferences/tests/test_scoped_preferences.py

@@ -42,14 +42,18 @@ class ScopedPreferencesTestCase(PreferencesTestCase):
     def setUp(self):
         """ Prepares the test fixture before each test method is called. """
 
-        self.preferences = ScopedPreferences()
+        # A temporary directory that can safely be written to.
+        self.tmpdir = tempfile.mkdtemp()
+
+        self.preferences = ScopedPreferences(
+            application_preferences_filename=os.path.join(
+                self.tmpdir, "preferences.ini"
+            )
+        )
 
         # The filename of the example preferences file.
         self.example = os.fspath(files(PKG) / "example.ini")
 
-        # A temporary directory that can safely be written to.
-        self.tmpdir = tempfile.mkdtemp()
-
     def tearDown(self):
         """ Called immediately after each test method has been called. """