compare

fIn, _ := os.Open("testdata/fixture.txt")
defer fIn.Close()

fOut, _ := os.Create("testdata/actual.txt")
defer fOut.Close()

scanner := bufio.NewScanner(fIn)
scanner.Split(bufio.ScanLines)
writer := bufio.NewWriter(fOut)

lineNum := 0
for scanner.Scan() {
	line := scanner.Text()
	lineNum++
	fmt.Fprintf(writer, "%04d %s\n", lineNum, line)
}
writer.Flush()

mockT := testing.T{}
var t *testing.T = &mockT
compare.ExpectedActual(t, "testdata/expected.txt", "testdata/actual.txt")

Output:

0001 THE QUEEN'S TWIN AND OTHER STORIES
0002 BY SARAH ORNE JEWETT
0003 The coast of Maine was in former years brought so near to foreign
0004 shores by its busy fleet of ships that among the older men and women
0005 one still finds a surprising proportion of travelers.  Each
0006 seaward-stretching headland with its high-set houses, each island of a
0007 single farm, has sent its spies to view many a Land of Eshcol; one may
0008 see plain, contented old faces at the windows, whose eyes have looked
0009 at far-away ports and known the splendors of the Eastern world.  They
0010 shame the easy voyager of the North Atlantic and the Mediterranean;
0011 they have rounded the Cape of Good Hope and braved the angry seas of
0012 Cape Horn in small wooden ships; they have brought up their hardy boys
0013 and girls on narrow decks; they were among the last of the Northmen's
0014 children to go adventuring to unknown shores.  More than this one
0015 cannot give to a young State for its enlightenment; the sea captains
0016 and the captains' wives of Maine knew something of the wide world, and
0017 never mistook their native parishes for the whole instead of a part
0018 thereof; they knew not only Thomaston and Castine and Portland, but
0019 London and Bristol and Bordeaux, and the strange-mannered harbors of
0020 the China Sea.