Allow tiny sacct I/O size discrepancies

test12.2 was consistently failing on smd# cluster with tiny differences
  in the disk read and written. This change permits those tiny discrepancies
  to exist without failing the test.
  Here are the numbers, which are consistent:

  sacct --noheader -p --job=763.0 --format MaxDiskWrite,AveDiskWrite,MaxDiskRead,AveDiskRead
  10.00M|10.00M|10.03M|10.03M|
  (i.e. 0.3% discrepancy, up to 0.5% allowed with current code)