Distribute modifications properly in test_calc_modify.py

test_calc_modify.py test is used for testing the wiredtiger_calc_modify function with a different set of document sizes and random modifications in it, but the distribution of the modifications are not spread across the document properly because of wrong offset calculation.

offsets variable contains the offsets in the increasing order. These offset values are used to copy/delete the old data, deleting old data and accessing directly with increased offset leads to point to end of the old data directly and all the modifications are occurring directly at the end of the new data instead of spreading them.

It is better to use the difference between the offsets while accessing the old and new value will spread the changes evenly.