test: remove `sleep` usage in `test-nointerrupt.t` (issue6271)

We move from sleep based synchronisation to file creation based synchronisation.

Sleeps is the path to the dark side.
Sleeps leads to flakiness.
Flakiness leads to anger.
Anger leads to hate.
Hate leads to suffering.
16 jobs for topic/stable/issue6271 in 58 minutes and 19 seconds (queued for 4 minutes and 19 seconds)