[#651] shard/test: Speedup Shard_Delete

Signed-off-by: Alexander Chuprov <a.chuprov@yadro.com>

pkg/local_object_storage/shard/delete_test.go

@@ -61,6 +61,7 @@ func testShard(t *testing.T, hasWriteCache bool, payloadSize int) {
 	require.NoError(t, err)
 
 	if hasWriteCache {
+		sh.FlushWriteCache(context.Background(), FlushWriteCachePrm{ignoreErrors: false})
 		require.Eventually(t, func() bool {
 			_, err = sh.Delete(context.Background(), delPrm)
 			return err == nil