Fix addr/string type confusion in Broker::peers()

Broker::NetworkInfo$address is of type string. The Broker::peers() BiF treated
it as an address, which caused ZVal's type validation to flag a
mismatch and abort.

Includes a small tweak to the broker.unpeer test that makes it invoke
Broker::peers(), which breaks in absence of the fix.

testing/btest/broker/unpeer.zeek

@@ -43,6 +43,7 @@ event zeek_init()
 
 event Broker::peer_added(endpoint: Broker::EndpointInfo, msg: string)
     {
+    print fmt("peered, now %d peer(s)", |Broker::peers()|);
     schedule 2secs { print_something(1) };
     schedule 4secs { unpeer(endpoint) };
     }