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/Cargo.lock

@@ -4457,7 +4457,6 @@ dependencies = [
  "polkadot-node-primitives",
  "polkadot-node-subsystem",
  "polkadot-primitives",
- "polkadot-subsystem-test-helpers",
  "sc-network",
  "sp-core",
  "sp-runtime",
@@ -4498,7 +4497,6 @@ dependencies = [
  "polkadot-node-subsystem",
  "polkadot-primitives",
  "polkadot-statement-table",
- "polkadot-subsystem-test-helpers",
  "sc-client-api",
  "sc-keystore",
  "sp-api",
@@ -4549,12 +4547,14 @@ dependencies = [
 name = "polkadot-node-subsystem"
 version = "0.1.0"
 dependencies = [
+ "assert_matches",
  "async-trait",
  "derive_more 0.99.9",
  "futures 0.3.5",
  "futures-timer 3.0.2",
  "log 0.4.8",
  "parity-scale-codec",
+ "parking_lot 0.10.2",
  "pin-project",
  "polkadot-node-primitives",
  "polkadot-primitives",
@@ -4615,7 +4615,6 @@ dependencies = [
  "polkadot-node-primitives",
  "polkadot-node-subsystem",
  "polkadot-primitives",
- "polkadot-subsystem-test-helpers",
  "sc-network",
  "sp-core",
  "sp-runtime",
@@ -4970,7 +4969,6 @@ dependencies = [
  "polkadot-node-primitives",
  "polkadot-node-subsystem",
  "polkadot-primitives",
- "polkadot-subsystem-test-helpers",
  "sp-core",
  "sp-keyring",
  "sp-runtime",
@@ -4987,17 +4985,6 @@ dependencies = [
  "sp-core",
 ]
 
-[[package]]
-name = "polkadot-subsystem-test-helpers"
-version = "0.1.0"
-dependencies = [
- "async-trait",
- "futures 0.3.5",
- "parking_lot 0.10.2",
- "polkadot-node-subsystem",
- "sp-core",
-]
-
 [[package]]
 name = "polkadot-test-runtime"
 version = "0.8.17"