macOS needs even more extra time in test_blocking_lock_file

As seen in:
https://github.com/EliahKagan/GitPython/actions/runs/7056321920/attempts/1

Usually an extra_time of 0.12 (6x) is sufficient for both Windows
and macOS, but sometimes macOS needs even more, so this increases
it to 0.18 (9x) for macOS.

test/test_util.py

@@ -397,8 +397,10 @@ def test_blocking_lock_file(self):
         self.assertRaises(IOError, wait_lock._obtain_lock)
         elapsed = time.time() - start
         extra_time = 0.02
-        if os.name == "nt" or sys.platform == "cygwin" or sys.platform == "darwin":
-            extra_time *= 6  # NOTE: Indeterministic failures without this...
+        if os.name == "nt" or sys.platform == "cygwin":
+            extra_time *= 6  # Without this, we get indeterministic failures on Windows.
+        elif sys.platform == "darwin":
+            extra_time *= 9  # The situation on macOS is similar, but with more delay.
         self.assertLess(elapsed, wait_time + extra_time)
 
     def test_user_id(self):