Run cygpath tests on Cygwin, not native Windows

They were not running on Cygwin, because git.util.is_win is False
on Cygwin. They were running on native Windows, with a number of
them always failing; these failures had sometimes been obscured by
the --maxfail=10 that had formerly been used (from pyproject.toml).

Many of them (not all the same ones) fail on Cygwin, and it might
be valuable for cygpath to work on other platforms, especially
native Windows. But I think it still makes sense to limit the tests
to Cygwin at this time, because all the uses of cygpath in the
project are in code that only runs after a check that the platform
is Cygwin. Part of that check, as it is implemented, explicitly
excludes native Windows (is_win must be false).

test/test_util.py

@@ -9,7 +9,7 @@
 import sys
 import tempfile
 import time
-from unittest import mock, skipIf
+from unittest import mock, skipUnless
 from datetime import datetime
 
 import ddt
@@ -84,14 +84,14 @@ def setup(self):
             "array": [42],
         }
 
-    @skipIf(not is_win, "Paths specifically for Windows.")
+    @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.idata(_norm_cygpath_pairs + _unc_cygpath_pairs)
     def test_cygpath_ok(self, case):
         wpath, cpath = case
         cwpath = cygpath(wpath)
         self.assertEqual(cwpath, cpath, wpath)
 
-    @skipIf(not is_win, "Paths specifically for Windows.")
+    @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.data(
         (r"./bar", "bar"),
         (r".\bar", "bar"),
@@ -104,7 +104,7 @@ def test_cygpath_norm_ok(self, case):
         cwpath = cygpath(wpath)
         self.assertEqual(cwpath, cpath or wpath, wpath)
 
-    @skipIf(not is_win, "Paths specifically for Windows.")
+    @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.data(
         r"C:",
         r"C:Relative",
@@ -117,7 +117,7 @@ def test_cygpath_invalids(self, wpath):
         cwpath = cygpath(wpath)
         self.assertEqual(cwpath, wpath.replace("\\", "/"), wpath)
 
-    @skipIf(not is_win, "Paths specifically for Windows.")
+    @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.idata(_norm_cygpath_pairs)
     def test_decygpath(self, case):
         wpath, cpath = case