Fix tracking validator set in ImOnline (#3596)

* Use session::validators instead of staking::current_elected

* Basic test framework.

* Initialize validators, attempt to heartbeat.

* Use dummy crypto for im-online testing.

* Remove printlns.

* Finish test, make it invalid.

* Add reporting test.

* Finalize the test.

* Remove dumbness.

* Updates.

* Update AuRa

* Update srml/im-online/src/tests.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Derive Ord

* Add some more tests.

* Remove stray todo.

* Bump runtime version.

* Bump impl-trait-for-tuples.

* Enforce new version of trait-for-tuples.

substrate/srml/authority-discovery/src/lib.rs

@@ -137,7 +137,6 @@ mod tests {
 	use sr_primitives::testing::{Header, UintAuthorityId};
 	use sr_primitives::traits::{ConvertInto, IdentityLookup, OpaqueKeys};
 	use sr_primitives::Perbill;
-	use sr_staking_primitives::CurrentElectedSet;
 	use support::{impl_outer_origin, parameter_types};
 
 	type AuthorityDiscovery = Module<Test>;
@@ -149,13 +148,6 @@ mod tests {
 
 	type AuthorityId = im_online::sr25519::AuthorityId;
 
-	pub struct DummyCurrentElectedSet<T>(std::marker::PhantomData<T>);
-	impl<T> CurrentElectedSet<T> for DummyCurrentElectedSet<T> {
-		fn current_elected_set() -> Vec<T> {
-			vec![]
-		}
-	}
-
 	pub struct TestOnSessionEnding;
 	impl session::OnSessionEnding<AuthorityId> for TestOnSessionEnding {
 		fn on_session_ending(_: SessionIndex, _: SessionIndex) -> Option<Vec<AuthorityId>> {
@@ -189,7 +181,6 @@ mod tests {
 			UncheckedExtrinsic<(), im_online::Call<Test>, (), ()>,
 		>;
 		type ReportUnresponsiveness = ();
-		type CurrentElectedSet = DummyCurrentElectedSet<AuthorityId>;
 	}
 
 	pub type BlockNumber = u64;