Don't double-interrupt the test HTTP server

Copied from my PR to fix this double-interrupt race.

See ruby/net-http#197

test/mri/net/http/utils.rb

@@ -27,12 +27,14 @@ def initialize(config, &block)
     def start
       @thread = Thread.new do
         loop do
-          socket = @ssl_server ? @ssl_server.accept : @server.accept
+          socket = (@ssl_server || @server).accept
           run(socket)
         rescue
         ensure
-          socket.close if socket
+          socket&.close
         end
+      ensure
+        (@ssl_server || @server).close
       end
     end
 
@@ -42,7 +44,6 @@ def run(socket)
 
     def shutdown
       @thread&.kill
-      @server&.close
       @thread&.join
     end