tests: add write/fsync stall test for fileiotest
tests: add write/fsync stall test for fileiotest

Generate a write+fsync workload and report on stalls. Since we don't
yet have a good feeling for how many stalls are generated on different
kernels, let it always pass for now and just report the results.

scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar