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.