TEST: Use addCleanup in test_nbextensions.

This ensures that resources/patches created during setUp are torn down
correctly even if setUp fails.

notebook/tests/test_nbextensions.py

@@ -62,7 +62,15 @@ class TestInstallNBExtension(TestCase):
         return py3compat.cast_unicode(td.name)
 
     def setUp(self):
+        # Any TemporaryDirectory objects appended to this list will be cleaned
+        # up at the end of the test run.
         self.tempdirs = []
+
+        @self.addCleanup
+        def cleanup_tempdirs():
+            for d in self.tempdirs:
+                d.cleanup()
+
         self.src = self.tempdir()
         self.files = files = [
             pjoin(u'ƒile'),
@@ -75,25 +83,30 @@ class TestInstallNBExtension(TestCase):
             if not os.path.exists(parent):
                 os.makedirs(parent)
             touch(fullpath)
-        
+
         self.test_dir = self.tempdir()
         self.data_dir = os.path.join(self.test_dir, 'data')
         self.config_dir = os.path.join(self.test_dir, 'config')
         self.system_data_dir = os.path.join(self.test_dir, 'system_data')
         self.system_path = [self.system_data_dir]
         self.system_nbext = os.path.join(self.system_data_dir, 'nbextensions')
+
+        # Patch out os.environ so that tests are isolated from the real OS
+        # environment.
         self.patch_env = patch.dict('os.environ', {
             'JUPYTER_CONFIG_DIR': self.config_dir,
             'JUPYTER_DATA_DIR': self.data_dir,
         })
         self.patch_env.start()
-        self.patch_system_path = patch.object(nbextensions,
-            'SYSTEM_JUPYTER_PATH', self.system_path)
+        self.addCleanup(self.patch_env.stop)
+
+        # Patch out the system path os that we consistently use our own
+        # temporary directory instead.
+        self.patch_system_path = patch.object(
+            nbextensions, 'SYSTEM_JUPYTER_PATH', self.system_path
+        )
         self.patch_system_path.start()
-    
-    def tearDown(self):
-        self.patch_env.stop()
-        self.patch_system_path.stop()
+        self.addCleanup(self.patch_system_path.stop)
 
     def assert_dir_exists(self, path):
         if not os.path.exists(path):