debug/elf: deflake file_test.go

It is valid for io.Reader to return (n, io.EOF) where n is positive.
The unit test should not fail if io.EOF is returned when read until
the end.

Change-Id: I7b918e3cc03db8b90c8aa58f4c0f7806a1d4af7e
Reviewed-on: https://go-review.googlesource.com/21307Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/debug/elf/file_test.go

@@ -688,7 +688,7 @@ func TestCompressedSection(t *testing.T) {
 		if end > int64(len(buf)) {
 			end = int64(len(buf))
 		}
-		n, err := sf.Read(buf[pos:end])
+		n, err := io.ReadFull(sf, buf[pos:end])
 		if err != nil {
 			t.Fatal(err)
 		}