Approval voting full subsystem tests (#3391)

* node/approval-voting: Introduce Backend trait and Overlaybackend

This commit introduces a Backend trait and attempts to move away
from the Action model via an OverlayBackend as in the ChainSelection
subsystem.

* node/approval-voting: Add WriteOps for StoredBlockRange and BlocksAtHeight

* node/approval-voting: Add load_all_blocks to overlay

* node/approval-voting: Get all module tests to pass.

This commit modifies all tests to ensure tests are passing.

* node/approval-voting: Address oversights in the previous commit

This commit addresses some oversights in the prior commit.

1. Inner errors in backend.write were swallowed
2. One-off write functions removed to avoid useless abstraction
3. Touch-ups in general

* node/approval-voting: Move from TestDB to dyn KeyValueDB

This commit removes the TestDB from tests.rs and replaces it with
an in-memory kvdb.

* node/approval-voting: Address feedback

* node/approval-voting: Add license to ops.rs

* node/approval-voting: Address second-pass feedback

* Add TODO

* node/approval-voting: Bump spec_version

* node/approval-voting: Address final comments.

* node/approval-voting: Introduce framework for full subsystem tests

* node/approval-voting: Introduce basic tests to attempt to provide
coverage via full subsystem tests

* node/approval-voting: Introduce Chainbuilder

node/core/approval-voting/Cargo.toml

@@ -42,3 +42,4 @@ maplit = "1.0.2"
 polkadot-node-subsystem-test-helpers = { path = "../../subsystem-test-helpers" }
 assert_matches = "1.4.0"
 kvdb-memorydb = "0.10.0"
+rand = "0.8"

node/core/approval-voting/src/import.rs

@@ -557,19 +557,19 @@ pub(crate) async fn handle_new_head<'a>(
 }
 
 #[cfg(test)]
-mod tests {
+pub(crate) mod tests {
 	use super::*;
 	use crate::approval_db::v1::DbBackend;
 	use polkadot_node_subsystem_test_helpers::make_subsystem_context;
 	use polkadot_node_primitives::approval::{VRFOutput, VRFProof};
 	use polkadot_primitives::v1::{SessionInfo, ValidatorIndex};
 	use polkadot_node_subsystem::messages::AllMessages;
 	use sp_core::testing::TaskExecutor;
-	use sp_runtime::{Digest, DigestItem};
-	use sp_consensus_babe::{
+	pub(crate) use sp_runtime::{Digest, DigestItem};
+	pub(crate) use sp_consensus_babe::{
 		Epoch as BabeEpoch, BabeEpochConfiguration, AllowedSlots,
 	};
-	use sp_consensus_babe::digests::{CompatibleDigestItem, PreDigest, SecondaryVRFPreDigest};
+	pub(crate) use sp_consensus_babe::digests::{CompatibleDigestItem, PreDigest, SecondaryVRFPreDigest};
 	use sp_keyring::sr25519::Keyring as Sr25519Keyring;
 	use assert_matches::assert_matches;
 	use merlin::Transcript;
@@ -650,7 +650,7 @@ mod tests {
 	}
 
 	// used for generating assignments where the validity of the VRF doesn't matter.
-	fn garbage_vrf() -> (VRFOutput, VRFProof) {
+	pub(crate) fn garbage_vrf() -> (VRFOutput, VRFProof) {
 		let key = Sr25519Keyring::Alice.pair();
 		let key: &schnorrkel::Keypair = key.as_ref();
 

node/core/approval-voting/src/lib.rs

@@ -85,9 +85,13 @@ use crate::backend::{Backend, OverlayedBackend};
 #[cfg(test)]
 mod tests;
 
+#[cfg(test)]
+mod old_tests;
+
 const APPROVAL_SESSIONS: SessionIndex = 6;
 const APPROVAL_CHECKING_TIMEOUT: Duration = Duration::from_secs(120);
 const APPROVAL_CACHE_SIZE: usize = 1024;
+const TICK_TOO_FAR_IN_FUTURE: Tick = 20; // 10 seconds.
 const LOG_TARGET: &str = "parachain::approval-voting";
 
 /// Configuration for the approval voting subsystem
@@ -1420,9 +1424,8 @@ fn check_and_import_assignment(
 	assignment: IndirectAssignmentCert,
 	candidate_index: CandidateIndex,
 ) -> SubsystemResult<(AssignmentCheckResult, Vec<Action>)> {
-	const TICK_TOO_FAR_IN_FUTURE: Tick = 20; // 10 seconds.
-
 	let tick_now = state.clock.tick_now();
+
 	let block_entry = match db.load_block_entry(&assignment.block_hash)? {
 		Some(b) => b,
 		None => return Ok((AssignmentCheckResult::Bad(