Testing a function in golang

So I've decided to write test for my function in golang. The function itself looks like this:

func Insert(slice []int, element int, index int) []int {
    n := len(slice)
    slice = slice[:(n + 1)]
    for i := index; i < n; i++ {
        slice[i+1] = slice[i]
    }
    slice[index] = element
    return slice
} 

So it takes a slice, extends its length by 1 and inserts an element at given index. Now before I call it I have to do 2 things:

size := somevalue
array := make([]int, size, size+1)

and then f.e array = Insert(array, 1, len(array)/2. Now I wrote insert_test.go:

package sdizo

import "testing"

func benchmarkInsert(size int, b *testing.B) {  
    b.ResetTimer()
    for n := 0; n < b.N; n++ {
        array := make([]int, size, size+1)
        array = Insert(array, 1, len(array)/2)
    }
}
func BenchmarkInsert10k(b *testing.B) { benchmarkInsert(10000, b) }
func BenchmarkInsert20k(b *testing.B) { benchmarkInsert(20000, b) }
func BenchmarkInsert30k(b *testing.B) { benchmarkInsert(30000, b) }
func BenchmarkInsert40k(b *testing.B) { benchmarkInsert(40000, b) }
func BenchmarkInsert50k(b *testing.B) { benchmarkInsert(50000, b) }

The thing is, there are 2 operations in testing loop. I cannot move the make() above the loop, cuz it has to make a new array everytime it tries to insert something to it (don't want to mess with capacity). It works, it gives me output, but I am curious if this make() doesn't mess up with time measurement, and I would like to ask you if I can somehow measure the time only for Insert()

P.S Is there a convenient way to write down the results of benchmark test to a file?