Remove sleep from test_slow_retrieval_attack.py

Now, after we can use wait_for_server and the retry mechanism
of TestServerProcess in utils.py we no longer need to use
sleep in this test file.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tests/test_slow_retrieval_attack.py

@@ -172,14 +172,6 @@ def setUp(self):
 
     logger.info('Slow Retrieval Server process started.')
 
-    # NOTE: Following error is raised if a delay is not long enough:
-    # <urlopen error [Errno 111] Connection refused>
-    # or, on Windows:
-    # Failed to establish a new connection: [Errno 111] Connection refused'
-    # 1s led to occasional failures in automated builds on AppVeyor, so
-    # increasing this to 3s, sadly.
-    time.sleep(3)
-
     url_prefix = 'http://localhost:' \
       + str(self.server_process_handler.port) + repository_basepath