Measure decompression time in compressio decompression benchmarks.

IIUC, Go sets testing.b.N by running benchmarks repeatedly with increasing
values of b.N until a "reasonable" benchmark run time is achieved. Before this
CL, decompression benchmarks measure the time to perform *compression* once
while scaling the number of decompressions with b.N, making the Go-observed
benchmark time independent of b.N and causing Go to increase b.N to
unpredictable and unreasonable amounts:

```
BenchmarkDecompressNoHash1M
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 821.474577ms, ratio 0.75, decompression time 1.161417487s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 526.175362ms, ratio 0.75, decompression time 1.937750371s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 767.402422ms, ratio 0.75, decompression time 2.156883246s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 779.148676ms, ratio 0.75, decompression time 2.396898003s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 577.573005ms, ratio 0.75, decompression time 3.266100928s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 524.66472ms, ratio 0.75, decompression time 5.922212382s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 714.897653ms, ratio 0.75, decompression time 13.105200153s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 768.187436ms, ratio 0.75, decompression time 19.574360494s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 625.937757ms, ratio 0.75, decompression time 30.097482785s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 767.470509ms, ratio 0.75, decompression time 56.130162322s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 527.026934ms, ratio 0.75, decompression time 1m36.04235679s
    pkg/compressio/compressio_test.go:152: compress=false, hash=false, len(data)=838860800, blockSize=1048576: compression time 943.736532ms, ratio 0.75, decompression time 3m15.294992151s
panic: SIGTERM
```
PiperOrigin-RevId: 615249406

Author: nixprime

pkg/compressio/compressio_test.go

@@ -219,31 +219,30 @@ func benchmark(b *testing.B, compress bool, hash bool, blockSize uint32) {
 	b.StopTimer()
 	b.SetBytes(benchDataSize)
 	data := initTest(b, benchDataSize)
-	compIters := b.N
-	decompIters := b.N
-	if compress {
-		decompIters = 0
-	} else {
-		compIters = 0
-	}
 	key := hashKey
 	if !hash {
 		key = nil
 	}
-	doTest(b, testOpts{
-		Name:         fmt.Sprintf("compress=%t, hash=%t, len(data)=%d, blockSize=%d", compress, hash, len(data), blockSize),
-		Data:         data,
-		PreCompress:  b.StartTimer,
-		PostCompress: b.StopTimer,
+	opts := testOpts{
+		Name: fmt.Sprintf("compress=%t, hash=%t, len(data)=%d, blockSize=%d", compress, hash, len(data), blockSize),
+		Data: data,
 		NewWriter: func(b *bytes.Buffer) (io.Writer, error) {
 			return NewWriter(b, key, blockSize, flate.BestSpeed)
 		},
 		NewReader: func(b *bytes.Buffer) (io.Reader, error) {
 			return NewReader(b, key)
 		},
-		CompressIters:   compIters,
-		DecompressIters: decompIters,
-	})
+	}
+	if compress {
+		opts.PreCompress = b.StartTimer
+		opts.PostCompress = b.StopTimer
+		opts.CompressIters = b.N
+	} else {
+		opts.PreDecompress = b.StartTimer
+		opts.PostDecompress = b.StopTimer
+		opts.DecompressIters = b.N
+	}
+	doTest(b, opts)
 }
 
 func BenchmarkCompressNoHash64K(b *testing.B) {