compress/flate: fix a fmt.Fprintf style nit in a test.

It's not a big deal (the for loop drops from 130-ish to 120-ish
milliseconds for me) but it's not a big change either.

Change-Id: I161a49caab5cae5a2b87866ed1dfb93627be8013
Reviewed-on: https://go-review.googlesource.com/22110Reviewed-by: Klaus Post <klauspost@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/compress/flate/writer_test.go

@@ -91,7 +91,7 @@ func TestWriteError(t *testing.T) {
 		n *= 4
 	}
 	for i := 0; i < n; i++ {
-		buf.WriteString(fmt.Sprintf("asdasfasf%d%dfghfgujyut%dyutyu\n", i, i, i))
+		fmt.Fprintf(buf, "asdasfasf%d%dfghfgujyut%dyutyu\n", i, i, i)
 	}
 	in := buf.Bytes()
 	// We create our own buffer to control number of writes.