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/subsystem/Cargo.toml

@@ -13,6 +13,7 @@ futures-timer = "3.0.2"
 keystore = { package = "sc-keystore", git = "https://github.com/paritytech/substrate", branch = "master" }
 log = "0.4.8"
 parity-scale-codec = "1.3.0"
+parking_lot = { version = "0.10.0", optional = true }
 pin-project = "0.4.22"
 polkadot-node-primitives = { path = "../primitives" }
 polkadot-primitives = { path = "../../primitives" }
@@ -20,3 +21,12 @@ polkadot-statement-table = { path = "../../statement-table" }
 sc-network = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sp-core = { git = "https://github.com/paritytech/substrate", branch = "master" }
 streamunordered = "0.5.1"
+
+[dev-dependencies]
+assert_matches = "1.3.0"
+async-trait = "0.1"
+futures = { version = "0.3.5", features = ["thread-pool"] }
+parking_lot = "0.10.0"
+
+[features]
+test-helpers = [ "parking_lot" ]