core: fix blockchain goroutine leaks in tests

core/chain_makers_test.go

@@ -81,7 +81,10 @@ func ExampleGenerateChain() {
 
 	// Import the chain. This runs all block validation rules.
 	evmux := &event.TypeMux{}
+
 	blockchain, _ := NewBlockChain(db, gspec.Config, ethash.NewFaker(), evmux, vm.Config{})
+	defer blockchain.Stop()
+
 	if i, err := blockchain.InsertChain(chain); err != nil {
 		fmt.Printf("insert error (block %d): %v\n", chain[i].NumberU64(), err)
 		return