test_dns: Fix FD leak

The listening TCP socket is closed by `with_udp_and_tcp` helper, but
the connected socket is leaking.

```
Leaked file descriptor: TestResolvDNS#test_multiple_servers_with_timeout_and_truncated_tcp_fallback: 12 : #<TCPSocket:fd 12, AF_INET, 127.0.0.1, 50888>
COMMAND     PID     USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
ruby    3248055 chkbuild   12u  IPv4 112546322      0t0  TCP localhost:50888->localhost:40112 (CLOSE_WAIT)
```

For the purpose of the test case to simulate a timeout over TCP
transport, we have to delay closing this socket until the end the test
case.

Fixup: #50

test/resolv/test_dns.rb

@@ -758,7 +758,10 @@ def test_multiple_servers_with_timeout_and_truncated_tcp_fallback
           u1.send(msg[0...512], 0, client_address, client_port)
         end
 
-        tcp_server1_thread = Thread.new { t1.accept; t1.close }
+        tcp_server1_thread = Thread.new do
+          # Keep this socket open so that the client experiences a timeout
+          t1.accept
+        end
 
         tcp_server2_thread = Thread.new do
           ct = t2.accept
@@ -800,7 +803,7 @@ def test_multiple_servers_with_timeout_and_truncated_tcp_fallback
           ct.send(msg, 0)
           ct.close
         end
-        result, = assert_join_threads([client_thread, udp_server1_thread, tcp_server1_thread, tcp_server2_thread])
+        result, _, tcp_server1_socket, = assert_join_threads([client_thread, udp_server1_thread, tcp_server1_thread, tcp_server2_thread])
         assert_instance_of(Array, result)
         assert_equal(50, result.length)
         result.each_with_index do |rr, i|
@@ -809,6 +812,8 @@ def test_multiple_servers_with_timeout_and_truncated_tcp_fallback
           assert_equal("192.0.2.#{i}", rr.address.to_s)
           assert_equal(3600, rr.ttl)
         end
+      ensure
+        tcp_server1_socket&.close
       end
     end
   end