operations: fix multithread copy test to use the correct modify window
operations: fix multithread copy test to use the correct modify window

In bde0334bd8e2c16d "operations: fix setting the timestamp on Windows
for multithread copy" the test for multithread copy failed to take
into account the modify window of the remote under test.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone