Make AllSubsystems usage easier in tests (#1794)

* Make `AllSubsystems` usage easier in tests

This makes the usage of `AllSubsystems` easier in tests by introducing
new methods.

- `dummy` initializes `AllSubsystems` with all systems set to dummy
- `replace_*` to replace any subsystem

Besides that this pr adds a `ForwardSubsystem` that is also useful for
tests. This subsystem will forward all incoming messages to the given channel.

* Update node/overseer/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/subsystem/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/subsystem/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Move ForwardSubsystem and add a test

* Break some lines

Co-authored-by: Andronik Ordian <write@reusable.software>

polkadot/node/overseer/examples/minimal-example.rs

@@ -30,10 +30,7 @@ use kv_log_macro as log;
 use polkadot_primitives::v1::{BlockData, PoV};
 use polkadot_overseer::{Overseer, AllSubsystems};
 
-use polkadot_subsystem::{
-	Subsystem, SubsystemContext, DummySubsystem,
-	SpawnedSubsystem, FromOverseer,
-};
+use polkadot_subsystem::{Subsystem, SubsystemContext, SpawnedSubsystem, FromOverseer};
 use polkadot_subsystem::messages::{
 	CandidateValidationMessage, CandidateBackingMessage, AllMessages,
 };
@@ -141,23 +138,9 @@ fn main() {
 			Delay::new(Duration::from_secs(1)).await;
 		});
 
-		let all_subsystems = AllSubsystems {
-			candidate_validation: Subsystem2,
-			candidate_backing: Subsystem1,
-			candidate_selection: DummySubsystem,
-			statement_distribution: DummySubsystem,
-			availability_distribution: DummySubsystem,
-			bitfield_signing: DummySubsystem,
-			bitfield_distribution: DummySubsystem,
-			provisioner: DummySubsystem,
-			pov_distribution: DummySubsystem,
-			runtime_api: DummySubsystem,
-			availability_store: DummySubsystem,
-			network_bridge: DummySubsystem,
-			chain_api: DummySubsystem,
-			collation_generation: DummySubsystem,
-			collator_protocol: DummySubsystem,
-		};
+		let all_subsystems = AllSubsystems::<()>::dummy()
+			.replace_candidate_validation(Subsystem2)
+			.replace_candidate_backing(Subsystem1);
 		let (overseer, _handler) = Overseer::new(
 			vec![],
 			all_subsystems,