[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/environment.rs

@@ -118,6 +118,7 @@ fn new_runtime() -> tokio::runtime::Runtime {
 		.thread_name("subsystem-bench")
 		.enable_all()
 		.thread_stack_size(3 * 1024 * 1024)
+		.worker_threads(4)
 		.build()
 		.unwrap()
 }