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>