all: get rid of error when creating memory database (#16716)

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition

tests/vm_test_util.go

@@ -79,8 +79,7 @@ type vmExecMarshaling struct {
 }
 
 func (t *VMTest) Run(vmconfig vm.Config) error {
-	db, _ := ethdb.NewMemDatabase()
-	statedb := MakePreState(db, t.json.Pre)
+	statedb := MakePreState(ethdb.NewMemDatabase(), t.json.Pre)
 	ret, gasRemaining, err := t.exec(statedb, vmconfig)
 
 	if t.json.GasRemaining == nil {