bpo-33944: site: Add site-packages tracing in verbose mode (GH-12110)

diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index 957e7a4..9f4a8bc 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -13,6 +13,7 @@
 import builtins
 import encodings
 import glob
+import io
 import os
 import re
 import shutil
@@ -320,6 +321,14 @@
             mock_addsitedir.assert_not_called()
             self.assertFalse(known_paths)
 
+    def test_trace(self):
+        message = "bla-bla-bla"
+        for verbose, out in (True, message + "\n"), (False, ""):
+            with mock.patch('sys.flags', mock.Mock(verbose=verbose)), \
+                    mock.patch('sys.stderr', io.StringIO()):
+                site._trace(message)
+                self.assertEqual(sys.stderr.getvalue(), out)
+
 
 class PthFile(object):
     """Helper class for handling testing of .pth files"""