fix: FS watcher when file does not exist yet (#18492)

The initial goal of this PR was to stabilise the test
`fs_watch_allows_missing_file_targets`. After further investigation, it
turns out that this test was always failing and the unstability was
coming from a race between timeouts mostly

The goal of the test was to test what happens if a notifier gets
subscribed while a file does not exist yet. But actually the main code
was broken and in case of a file not existing yet, the notifier used to
never notify anything (even if the file ended up being created)

This PR fixes the main code (and the test). For this, we basically watch
the sup-directory when a file does not exist and refresh on it when the
files gets created

Author: jif-oai