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/utils/frame/benchmarking-cli/src/lib.rs

@@ -104,6 +104,7 @@ impl BenchmarkCmd {
 		let executor = NativeExecutor::<ExecDispatch>::new(
 			wasm_method,
 			None, // heap pages
+			2, // The runtime instances cache size.
 		);
 
 		let result = StateMachine::<_, _, NumberFor<BB>, _>::new(