Add test suite and minor refinements to the utility subsystem (#1403)

* get conclude signal working properly; don't allocate a vector

* wip: add test suite / example / explanation for using utility subsystem

Unfortunately, the test fails right now for reasons which seem
very odd. Just have to keep poking at it.

* explicitly import everything

* fix subsystem-util test

The root problem here was two-fold:

- there was a circular dependency from subsystem -> test-helpers/subsystem ->
  subsystem
- cfg(test) doesn't propagate between crates

The solution: move the subsystem test helpers into a sub-module
within subsystem. Publicly export them from the previous location
so no other code breaks.

Doing this has an additional benefit: it ensures that no production
code can ever accidentally use the subsystem helpers, as they are compile-
gated on cfg(test).

* fully commit to moving test helpers into a subsystem module

* add some more tests

* get rid of log tests in favor of real error forwarding

It's not obvious whether we'll ever really want to chase down
these errors outside a testing context, but having the capability
won't hurt.

* fix issue which caused test to hang on osx

* only require that job errors are PartialEq when testing

also fix polkadot-node-core-backing tests

* get rid of any notion of partialeq

* rethink testing

Combine tests of starting and stopping job: leaving a test executor
with a job running was pretty clearly the cause of the sometimes-hang.

Also, add a timeout so tests _can't_ hang anymore; they just fail
after a while.

* rename fwd_errors -> forward_errors

* warn on error propagation failure

* fix unused import leftover from merge

* derive eq for subsystemerror

polkadot/node/network/bridge/src/lib.rs

@@ -531,7 +531,7 @@ mod tests {
 	use assert_matches::assert_matches;
 
 	use polkadot_subsystem::messages::{StatementDistributionMessage, BitfieldDistributionMessage};
-	use subsystem_test::{SingleItemSink, SingleItemStream};
+	use polkadot_subsystem::test_helpers::{SingleItemSink, SingleItemStream};
 
 	// The subsystem's view of the network - only supports a single call to `event_stream`.
 	struct TestNetwork {
@@ -550,7 +550,7 @@ mod tests {
 		TestNetwork,
 		TestNetworkHandle,
 	) {
-		let (net_tx, net_rx) = subsystem_test::single_item_sink();
+		let (net_tx, net_rx) = polkadot_subsystem::test_helpers::single_item_sink();
 		let (action_tx, action_rx) = mpsc::unbounded();
 
 		(
@@ -631,13 +631,13 @@ mod tests {
 
 	struct TestHarness {
 		network_handle: TestNetworkHandle,
-		virtual_overseer: subsystem_test::TestSubsystemContextHandle<NetworkBridgeMessage>,
+		virtual_overseer: polkadot_subsystem::test_helpers::TestSubsystemContextHandle<NetworkBridgeMessage>,
 	}
 
 	fn test_harness<T: Future<Output=()>>(test: impl FnOnce(TestHarness) -> T) {
 		let pool = sp_core::testing::SpawnBlockingExecutor::new();
 		let (network, network_handle) = new_test_network();
-		let (context, virtual_overseer) = subsystem_test::make_subsystem_context(pool);
+		let (context, virtual_overseer) = polkadot_subsystem::test_helpers::make_subsystem_context(pool);
 
 		let network_bridge = run_network(
 			network,