client: add a block blacklist extension (#4544)

* client: add a block blacklist extension

* test-utils: fix client construction

* client: fix rustdoc test

substrate/test-utils/client/src/lib.rs

@@ -204,6 +204,7 @@ impl<Executor, Backend, G: GenesisInit> TestClientBuilder<Executor, Backend, G>
 			executor,
 			storage,
 			Default::default(),
+			Default::default(),
 			ExecutionExtensions::new(
 				self.execution_strategies,
 				self.keystore.clone(),