✅ Add mutex to FakeServer::Connection#close

I believe this was getting triggered from multiple threads:
* from `FakeServer#shutdown` (which already has a mutex of its own)
  * called in ensure clause in `FakeServer#run` (from the server thread)
  * called directly by the tests (from the main thread)
* from the ensure clause in `Connection#run` (runs in the server thread)

So the main thread could call `FakeServer#shutdown` while the server
thread is handling the ensure clause in `Connection#run`.  Adding a
mutex to `Connection#close` should fix the flaky tests.

Author: nevans

test/net/imap/fake_server/connection.rb

@@ -15,6 +15,7 @@ def initialize(server, tcp_socket:)
       @reader = CommandReader.new  socket
       @writer = ResponseWriter.new socket, config: config, state: state
       @router = CommandRouter.new  writer, config: config, state: state
+      @mutex  = Thread::Mutex.new
     end
 
     def commands; state.commands end
@@ -31,11 +32,13 @@ def run
     end
 
     def close
-      unless state.logout?
-        state.logout
-        writer.bye
+      @mutex.synchronize do
+        unless state.logout?
+          state.logout
+          writer.bye
+        end
+        socket&.close unless socket&.closed?
       end
-      socket&.close unless socket&.closed?
     end
 
     private