As @Mr_Pink pointed out, I had to use the %q verb in my test for getting output with line breaks character \n printed out:

Test

func TestGuideBoardRender(t *testing.T) {
    var want string
    rendered := renderGuideBoard()
    want = "1  2  3  4  5\n6  7  8  9  10\n11 12 13 14 15\n16 17 18 19 20\n21 22 23 24 25"
    if rendered != want {
        t.Errorf("Guiding board values aren't right\ngot:\n\n%q\n\nwant:\n\n%q", rendered, want)
    }
}