Fix race condition in test election_scheduler.no_vacancy

Blocks send and receive are not confirmed for sure since nothing waits for
their confirmation. If they do not confirm in time, then one of the blocks
send or receive will take the only available place in the AEC and not
block2 as expected.

nano/core_test/election_scheduler.cpp

@@ -106,6 +106,8 @@ TEST (election_scheduler, no_vacancy)
 	ASSERT_EQ (nano::block_status::progress, node.process (receive));
 	node.process_confirmed (nano::election_status{ receive });
 
+	ASSERT_TIMELY (5s, nano::test::confirmed (node, { send, receive }));
+
 	// Second, process two eligible transactions
 	auto block1 = builder.make_block ()
 				  .account (nano::dev::genesis_key.pub)