client/finality-grandpa: Add regression test for observer polling network (was #4778) (#4795)

* client/finality-grandpa: Add regression test observer polling network

Ensure `Future` implementation of `ObserverWork` is polling its
`NetworkBridge`. Regression test for bug introduced in d4fbb897 and
fixed in 7d58ceeb

.

When polled, `NetworkBridge` forwards reputation change requests from
the `GossipValidator` to the underlying `dyn Network`. This test
triggers a reputation change by calling `GossipValidator::validate` with
an invalid gossip message. After polling the `ObserverWork` which should
poll the `NetworkBridge`, the reputation change should be forwarded to
the test network.

* Nits

Co-authored-by: Max Inden <[email protected]>