Explicitly check type of GetPeerInfo vector element

The `GetPeerInfo` type wraps a vector of `PeerInfo` types. In such
circumstances when we re-export the outer type super easy to either
forget to re-export the inner type. Currently most of our tests only
test the outer type.

Add a local variable assignment that explicitly uses the inner type,
there by proving all the re-exports are correct.

Tested by running v21 tests _without_ the re-export bug found in
review. And indeed this new test finds it.

Author: tcharding

integration_test/tests/network.rs

@@ -107,6 +107,10 @@ fn get_peer_info_three_node_network() {
     node2.mine_a_block();
     node3.mine_a_block();
 
+    let json: GetPeerInfo = node1.client.get_peer_info().expect("getpeerinfo");
+    // This verifies that we re-exported the correct `PeerInfo` type at the module level.
+    let _: PeerInfo = json.0[0];
+
     // FIXME: Fails if we use equal to 2 ???
     assert!(node1.peers_connected() >= 1);
     assert!(node2.peers_connected() >= 1);