chore: coding style (#3959)

core/mr/mapreduce_fuzzcase_test.go

@@ -20,7 +20,7 @@ import (
 // If Fuzz stuck, we don't know why, because it only returns hung or unexpected,
 // so we need to simulate the fuzz test in test mode.
 func TestMapReduceRandom(t *testing.T) {
-	rand.Seed(time.Now().UnixNano())
+	rand.NewSource(time.Now().UnixNano())
 
 	const (
 		times  = 10000