Prevent starting multiple Capybara proxy servers

This code is run every time a new Capybara browser session is started.
Running this code multiple times causes subsequent feature specs to slow
down considerably.

spec/support/capybara.rb

@@ -89,11 +89,13 @@
     options.add_preference("download.prompt_for_download", false)
   end
-  # Set up a proxy server to block all external traffic.
-  @blackhole_tcp_server = TCPServer.new(0)
-  Thread.new do
-    loop do
-      Thread.start(@blackhole_tcp_server.accept, &:close)
+  if @blackhole_tcp_server.nil?
+    # Set up a proxy server to block all external traffic.
+    @blackhole_tcp_server = TCPServer.new(0)
+    Thread.new do
+      loop do
+        Thread.start(@blackhole_tcp_server.accept, &:close)
+      end
     end
   end