tests: fstream: convert to using tmp_dir
tests: fstream: convert to using tmp_dir

Don't create temporary files in the current directory,
but rather in a temporary directory.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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