[subsystem-benchmarks] Fix availability-write regression tests (#3698)

Adds availability-write regression tests.
The results for the `availability-distribution` subsystem are volatile,
so I had to reduce the precision of the test.

polkadot/node/subsystem-bench/src/lib/mock/runtime_api.rs

@@ -36,6 +36,7 @@ use std::collections::HashMap;
 const LOG_TARGET: &str = "subsystem-bench::runtime-api-mock";
 
 /// Minimal state to answer requests.
+#[derive(Clone)]
 pub struct RuntimeApiState {
 	// All authorities in the test,
 	authorities: TestAuthorities,
@@ -49,6 +50,7 @@ pub struct RuntimeApiState {
 }
 
 /// A mocked `runtime-api` subsystem.
+#[derive(Clone)]
 pub struct MockRuntimeApi {
 	state: RuntimeApiState,
 	config: TestConfiguration,