Use CLI to configure max instances cache (#5177)

* Use CLI to configure max instances cache

* Fix tests

* Move default value into CLI

* Use SmallVec

* Apply review comments

* Get rid of `SmallVec`

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/service/test/src/lib.rs

@@ -210,6 +210,7 @@ fn node_config<G, E: Clone> (
 		dev_key_seed: key_seed,
 		tracing_targets: None,
 		tracing_receiver: Default::default(),
+		max_runtime_instances: 8,
 	}
 }