Fix test_blocking_lock_file for cygwin

This permits the longer delay in test_blocking_lock_file, which was
already allowed for native Windows, on Cygwin, which needs it too.
That makes the Cygwin xfail no longer needed, so this removes that.

It also updates the comments to avoid implying that the need for
the delay is AppVeyor-specific (it seems needed on CI and locally).

test/test_util.py

@@ -12,7 +12,6 @@
 from unittest import mock, skipIf
 from datetime import datetime
 
-import pytest
 import ddt
 
 from git.cmd import dashify
@@ -156,11 +155,6 @@ def test_lock_file(self):
         lock_file._obtain_lock_or_raise()
         lock_file._release_lock()
 
-    @pytest.mark.xfail(
-        sys.platform == "cygwin",
-        reason="Cygwin fails here for some reason, always",
-        raises=AssertionError
-    )
     def test_blocking_lock_file(self):
         my_file = tempfile.mktemp()
         lock_file = BlockingLockFile(my_file)
@@ -173,9 +167,8 @@ def test_blocking_lock_file(self):
         self.assertRaises(IOError, wait_lock._obtain_lock)
         elapsed = time.time() - start
         extra_time = 0.02
-        if is_win:
-            # for Appveyor
-            extra_time *= 6  # NOTE: Indeterministic failures here...
+        if is_win or sys.platform == "cygwin":
+            extra_time *= 6  # NOTE: Indeterministic failures without this...
         self.assertLess(elapsed, wait_time + extra_time)
 
     def test_user_id(self):