Merge branch 'overlap' of https://github.com/bvanassche/fio

* 'overlap' of https://github.com/bvanassche/fio:
  Add a test for serialize_overlap=1
  Enable error checking for the mutex that serializes overlapping I/O
  Fix a potentially infinite loop in check_overlap()
  t/run-fio-tests.py: Increase IOPS tolerance further
  .appveyor.yml: Select the latest software image