tests: fix shell errors in detach tests

tests: fix shell errors in detach tests

The current detach test code does:
	set -e
	...
	cleanup() {
		set +e
		kill ...
		wait ...
	}
	...
	cleanup
	exit 0

The problem is that while `set -e` is disabled for the body of the
cleanup function, it isn't necessarily disabled in the caller scope.
So if the return value of the cleanup function (`wait` in this case)
is non-zero, the script ends up failing overall.

Add an explicit return 0 to the cleanup function so that we don't kill
the overall test pipeline.

* tests/detach-running.test (cleanup): Add return 0.
* tests/detach-sleeping.test (cleanup): Likewise.
* tests/detach-stopped.test (cleanup): Likewise.