Revert "union: disable PartialUploads on integration tests failures"

This reverts commit 9065e921c1.

It turns out the problem for the failing fs/sync tests was the
policies being different for search and create which meant that the
file was being created in one union branch but a diferent one was
found in another branch.

backend/union/union.go

@@ -902,7 +902,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		ReadMetadata:            true,
 		WriteMetadata:           true,
 		UserMetadata:            true,
-		// PartialUploads:          true // this causes the fs/sync tests to fail
+		PartialUploads:          true,
 	}).Fill(ctx, f)
 	canMove, slowHash := true, false
 	for _, f := range upstreams {