[#236] blobstor/test: Prefill storage in parallel in read benchmark

`blobovniczatree` takes a really long time to prefill, because each
batch takes at least 10ms, so for 10k iterations we have at least 100s of
prefill.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobstor/perf_test.go

@@ -11,6 +11,7 @@ import (
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/memstore"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/testutil"
 	"github.com/stretchr/testify/require"
+	"golang.org/x/sync/errgroup"
 )
 
 type storage struct {
@@ -103,18 +104,23 @@ func BenchmarkSubstorageReadPerf(b *testing.B) {
 				st := stEntry.open(b)
 
 				// Fill database
+				var errG errgroup.Group
 				for i := 0; i < tt.size; i++ {
 					obj := objGen.Next()
 					addr := testutil.AddressFromObject(b, obj)
-					raw, err := obj.Marshal()
-					require.NoError(b, err)
-					if _, err := st.Put(common.PutPrm{
-						Address: addr,
-						RawData: raw,
-					}); err != nil {
-						b.Fatalf("writing entry: %v", err)
-					}
+					errG.Go(func() error {
+						raw, err := obj.Marshal()
+						if err != nil {
+							return fmt.Errorf("marshal: %v", err)
+						}
+						_, err = st.Put(common.PutPrm{
+							Address: addr,
+							RawData: raw,
+						})
+						return err
+					})
 				}
+				require.NoError(b, errG.Wait())
 
 				// Benchmark reading
 				addrGen := tt.addrGen()