streamclone: remove sleep based "synchronisation" in tests
Sleep based test synchronisation does not work.
Variation in machine performance and load can make the two process miss their
windows. Instead we migrate to explicit signaling through the file system as
other tests file are using.
Differential Revision: https://phab.mercurial-scm.org/D10478
testing: add a `write_file` function
The function is kinda trivial, but having a simple function avoid silly mistake.
Differential Revision: https://phab.mercurial-scm.org/D10477