refactor: make fake graph config's picklable

Author: bhearsum

packages/pytest-taskgraph/src/pytest_taskgraph/fixtures/gen.py

@@ -62,8 +62,13 @@ def _load_kinds(self, graph_config, target_kind=None):
             yield FakeKind(kind_name, "/fake", config, graph_config)
 
 
+class FakeGraphConfig(GraphConfig):
+    def register(self):
+        pass
+
+
 def fake_load_graph_config(root_dir):
-    graph_config = GraphConfig(
+    graph_config = FakeGraphConfig(
         {
             "trust-domain": "test-domain",
             "taskgraph": {
@@ -103,7 +108,6 @@ def fake_load_graph_config(root_dir):
         },
         root_dir,
     )
-    graph_config.__dict__["register"] = lambda: None
     return graph_config