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/statement-distribution/src/lib.rs

@@ -1213,7 +1213,7 @@ mod tests {
 		};
 
 		let pool = sp_core::testing::SpawnBlockingExecutor::new();
-		let (mut ctx, mut handle) = subsystem_test::make_subsystem_context(pool);
+		let (mut ctx, mut handle) = polkadot_subsystem::test_helpers::make_subsystem_context(pool);
 		let peer = PeerId::random();
 
 		executor::block_on(async move {
@@ -1305,7 +1305,7 @@ mod tests {
 		].into_iter().collect();
 
 		let pool = sp_core::testing::SpawnBlockingExecutor::new();
-		let (mut ctx, mut handle) = subsystem_test::make_subsystem_context(pool);
+		let (mut ctx, mut handle) = polkadot_subsystem::test_helpers::make_subsystem_context(pool);
 
 		executor::block_on(async move {
 			let statement = {