go get -u github.com/andreyvit/diff

import (
    "strings"
    "testing"
    "github.com/andreyvit/diff"
)

const expected = `
...
`

func TestFoo(t *testing.T) {
    actual := Foo(...)
    if a, e := strings.TrimSpace(actual), strings.TrimSpace(expected); a != e {
        t.Errorf("Result not as expected:\n%v", diff.LineDiff(e, a))
    }
}