test_imaplib: reap_server() closes the server when done

Fix a ResourceWarning(unclosed socket). Patch written by Nadeem Vawda.
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index f9fbe96..acb89e3 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -135,6 +135,7 @@
     def reap_server(self, server, thread):
         if verbose: print("waiting for server")
         server.shutdown()
+        server.server_close()
         thread.join()
         if verbose: print("done")