approval-distribution: Fix handling of conclude (#5375)

After

https://github.com/paritytech/polkadot-sdk/commit/0636ffdc


approval-distribution did not terminate anymore if Conclude signal was
received.

This should have been caught by the subsystem tests, but it wasn't
because the subsystem is also exiting on error when the channels are
dropped so the test overseer was dropped which made the susbystem exit
and masked the problem.

This pr fixes both the test and the subsystem.

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>