[#139] test: Add test storage implementation

This aims to reduce the usage of chmod hackery to induce or simulate
OS-related failures.

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/local_object_storage/writecache/writecache.go

@@ -1,6 +1,7 @@
 package writecache
 
 import (
+	"os"
 	"sync"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
@@ -106,6 +107,7 @@ func New(opts ...Option) Cache {
 			maxCacheSize:    defaultMaxCacheSize,
 			maxBatchSize:    bbolt.DefaultMaxBatchSize,
 			maxBatchDelay:   bbolt.DefaultMaxBatchDelay,
+			openFile:        os.OpenFile,
 		},
 	}