[#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/flush_test.go

@@ -46,12 +46,13 @@ func TestFlush(t *testing.T) {
 		require.NoError(t, mb.Open(false))
 		require.NoError(t, mb.Init())
 
-		fsTree := fstree.New(
-			fstree.WithPath(filepath.Join(dir, "blob")),
-			fstree.WithDepth(0),
-			fstree.WithDirNameLen(1))
 		bs := blobstor.New(blobstor.WithStorages([]blobstor.SubStorage{
-			{Storage: fsTree},
+			{
+				Storage: fstree.New(
+					fstree.WithPath(filepath.Join(dir, "blob")),
+					fstree.WithDepth(0),
+					fstree.WithDirNameLen(1)),
+			},
 		}))
 		require.NoError(t, bs.Open(false))
 		require.NoError(t, bs.Init())
@@ -208,7 +209,7 @@ func TestFlush(t *testing.T) {
 
 				_, err = os.Stat(p) // sanity check
 				require.NoError(t, err)
-				require.NoError(t, os.Chmod(p, 0))
+				require.NoError(t, os.Truncate(p, 0)) // corrupt the file contents, so that it can't be unmarshalled
 			})
 		})
 		t.Run("fs, invalid object", func(t *testing.T) {