Fix unit test rpc.account_representative_set_epoch_2 (#3774)

This test has 2 problems:
* it has a race condition between upgrading to epoch2 and sending the rpc
  command to change the rep
* the rpc command is given a work value that is incorrect because the
  wrong root is used to calculate it, since the work value is meant to be
  insufficient, it works most of the time anyway, but there are times when
  the value becomes sufficient and then the test fails

Solutions
* wait until the epoch upgrades blocks are cemented before sending the rpc
* use the correct root (hash of epoch2 block) to calculate the work field

resolves #3770

nano/rpc_test/rpc.cpp

@@ -2719,16 +2719,21 @@ TEST (rpc, account_representative_set_work_disabled)
 	}
 }
 
-TEST (rpc, account_representative_set_epoch_2)
+TEST (rpc, account_representative_set_epoch_2_insufficient_work)
 {
 	nano::system system;
 	auto node = add_ipc_enabled_node (system);
+	system.wallet (0)->insert_adhoc (nano::dev::genesis_key.prv, false);
 
 	// Upgrade the genesis account to epoch 2
 	ASSERT_NE (nullptr, system.upgrade_genesis_epoch (*node, nano::epoch::epoch_1));
 	ASSERT_NE (nullptr, system.upgrade_genesis_epoch (*node, nano::epoch::epoch_2));
 
-	system.wallet (0)->insert_adhoc (nano::dev::genesis_key.prv, false);
+	// speed up the cementing process, otherwise the node waits for frontiers confirmation to notice the unconfirmed epoch blocks, which takes time
+	node->scheduler.activate (nano::dev::genesis_key.pub, node->store.tx_begin_read ());
+
+	// wait for the epoch blocks to be cemented
+	ASSERT_TIMELY (5s, node->get_confirmation_height (node->store.tx_begin_read (), nano::dev::genesis_key.pub) == 3);
 
 	auto target_difficulty = nano::dev::network_params.work.threshold (nano::work_version::work_1, nano::block_details (nano::epoch::epoch_2, false, false, false));
 	ASSERT_LT (node->network_params.work.entry, target_difficulty);
@@ -2744,7 +2749,8 @@ TEST (rpc, account_representative_set_epoch_2)
 	request.put ("representative", nano::keypair ().pub.to_account ());
 
 	// Test that the correct error is given if there is insufficient work
-	auto insufficient = system.work_generate_limited (nano::dev::genesis->hash (), min_difficulty, target_difficulty);
+	auto latest = node->ledger.latest (node->store.tx_begin_read (), nano::dev::genesis_key.pub);
+	auto insufficient = system.work_generate_limited (latest, min_difficulty, target_difficulty);
 	request.put ("work", nano::to_string_hex (insufficient));
 	{
 		auto response (wait_response (system, rpc_ctx, request));