compress/bzip2: fix benchmark to actually measure decompression rate

Motivation:
* Previously, the size of the compressed data was used for metrics,
rather than the uncompressed size. This causes the library to appear
to perform poorly relative to C or other implementation. Switch it
to use the uncompressed size so that it matches how decompression
benchmarks are usually done (like in compress/flate). This also makes
it easier to compare bzip2 rates to other algorithms since they measure
performance in this way.
* Also, reset the timer after doing initialization work.

Change-Id: I32112c2ee8e7391e658c9cf31039f70a689d9b9d
Reviewed-on: https://go-review.googlesource.com/17611Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/compress/bzip2/bzip2_test.go

@@ -194,7 +194,17 @@ func benchmarkDecode(b *testing.B, testfile int) {
 	if err != nil {
 		b.Fatal(err)
 	}
-	b.SetBytes(int64(len(compressed)))
+
+	// Determine the uncompressed size of testfile.
+	uncompressedSize, err := io.Copy(ioutil.Discard, NewReader(bytes.NewReader(compressed)))
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.SetBytes(uncompressedSize)
+	b.ReportAllocs()
+	b.ResetTimer()
+
 	for i := 0; i < b.N; i++ {
 		r := bytes.NewReader(compressed)
 		io.Copy(ioutil.Discard, NewReader(r))