tests: tail/overlay-headers.sh: fix various issues

tests: tail/overlay-headers.sh: fix various issues

* tests/tail/overlay-headers.sh (cleanup_): Ensure we send SIGCONT
to the tail process, otherwise we would hang if the test is terminated
while the tail process is in stopped state.
(wait4stopped_): A new function to ensure tail is in the stopped state
before we start writing to the monitored files.
Also remove "---disable-inotify" from $fastpoll so we actually
test the inotify code (where supported).
Also remove the timeout(1) wrapper, so we actually suspend tail(1).

Reported by Bruno Haible on macOS 26