Changes

tests/osmo_io: Introduce small sleeps waiting for write completion

Sometimes test_file() fails during jenkins run because it finished the
main loop iterations without having received a write complete cb.
This may be due to filesystem delays due to system load, it's difficult
to say since it only happens sporadically.
Add some small sleeps under each loop iteration (which can end up
waiting up to 10s after full set of iterations) to make sure we receive
the write callback in a timely fashion.

Related: OS#6708
Change-Id: I637f9a5a5f633843b09d4677a0fff48bb75672e9