🧪 Enable retries of test_peercreds_unix_sock

This test was noticed to fail flakily with a socket read timeout under
slow envs like PyPy. The following attempt succeeds.

https://github.com/cherrypy/cheroot/actions/runs/4449504944/jobs/7813820004?pr=516#step:20:1844

cheroot/test/test_server.py

@@ -254,6 +254,7 @@ def peercreds_enabled_server(http_server, unix_sock_file):
 
 @unix_only_sock_test
 @non_macos_sock_test
+@pytest.mark.flaky(reruns=3, reruns_delay=2)
 def test_peercreds_unix_sock(http_request_timeout, peercreds_enabled_server):
     """Check that ``PEERCRED`` lookup works when enabled."""
     httpserver = peercreds_enabled_server