Add walk module to tests/__init__.py.

Change-Id: I50988ac9fa482809b9b96ac8be89c2e671f17b7a

dulwich/tests/__init__.py

@@ -95,6 +95,7 @@ def self_test_suite():
         'protocol',
         'repository',
         'server',
+        'walk',
         'web',
         ]
     module_names = ['dulwich.tests.test_' + name for name in names]