data: fix invalid trees used in test cases

data.TestCreateSnapshot which is used in particular by TestFindUsedBlobs
and TestFindUsedBlobs could generate trees with duplicate file names.
This is invalid and going forward will result in an error.

internal/data/testing.go

@@ -81,7 +81,7 @@ func (fs *fakeFileSystem) saveTree(ctx context.Context, uploader restic.BlobSave
 			id := fs.saveTree(ctx, uploader, treeSeed, depth-1)
 
 			node := &Node{
-				Name:    fmt.Sprintf("dir-%v", treeSeed),
+				Name:    fmt.Sprintf("dir-%v", i),
 				Type:    NodeTypeDir,
 				Mode:    0755,
 				Subtree: &id,
@@ -95,7 +95,7 @@ func (fs *fakeFileSystem) saveTree(ctx context.Context, uploader restic.BlobSave
 		fileSize := (maxFileSize / maxSeed) * fileSeed
 
 		node := &Node{
-			Name: fmt.Sprintf("file-%v", fileSeed),
+			Name: fmt.Sprintf("file-%v", i),
 			Type: NodeTypeFile,
 			Mode: 0644,
 			Size: uint64(fileSize),
@@ -106,7 +106,6 @@ func (fs *fakeFileSystem) saveTree(ctx context.Context, uploader restic.BlobSave
 	}
 
 	tree.Sort()
-
 	id, err := SaveTree(ctx, uploader, &tree)
 	if err != nil {
 		fs.t.Fatalf("SaveTree returned error: %v", err)