How can I use "testing.Benchmark" in a Benchmark function

my testing file

func BenchmarkA(b *testing.B) {
    res := testing.Benchmark(BenchB)
    if res.N <= 25{
        b.Fatalf("fail!")
    }

func BenchB(b *testing.B) {
for i := 0; i <= b.N; i++{
        url := "****"
        contentType := "application/x-www-form-urlencoded"
        data := strings.NewReader("****")
        _, err := http.Post(url, contentType, data)
        if err != nil {
            b.Fatal("fail ")
        }
    }
}

when i run

go test -bench=./router_test.go

I get a

fatal error: all goroutines are asleep - deadlock!

if i use "package main" and "func main{}", the program can run successfully.

what should i do next? (forgive my poor english)