Fix `clone_share_vm` test

This test is trickier than I thought.

The problem is the handling of cloned_file_data_fd_child when we restore captured state in Task::copy_state. For correct replay, that fd must refer to the opened cloned-data file for this task. But consider the case where this task has been cloned from a group-leader task holding its address space, but during recording this task had a different file descriptor table to that group-leader task (e.g. because this task was cloned with CLONE_VM but not CLONE_FILES); this task's cloned_file_data_fd_child and the group-leader's cloned_file_data_fd_child could be equal. During replay this task and the group-leader task currently always share file-descriptor tables so we can't have that same file descriptor value assigned to the two different files.

Start with the clone_share_vm test and the Task::copy_state path mentioned in the issue, tracing how cloned_file_data_fd_child is restored during replay. Done means replay assigns each task's descriptor to its own opened cloned-data file, including when the task and group leader had different descriptor tables during recording.