Move spawning tasks from thread pools to Service's TaskManager for block importing (#5647)

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

substrate/client/network/test/src/block_import.rs

@@ -84,7 +84,10 @@ fn async_import_queue_drops() {
 	// Perform this test multiple times since it exhibits non-deterministic behavior.
 	for _ in 0..100 {
 		let verifier = PassThroughVerifier(true);
-		let queue = BasicQueue::new(verifier, Box::new(substrate_test_runtime_client::new()), None, None);
+
+		let threads_pool = futures::executor::ThreadPool::new().unwrap();
+		let spawner = |future| threads_pool.spawn_ok(future);
+		let queue = BasicQueue::new(verifier, Box::new(substrate_test_runtime_client::new()), None, None, spawner);
 		drop(queue);
 	}
 }