Issue #7449, part 5: split Test.test_open() of ctypes/test/test_errno.py

* Split Test.test_open() in 2 functions: test_open() and test_thread_open()
 * Skip test_open() and test_thread_open() if we are unable to find the C
   library
 * Skip test_thread_open() if thread support is disabled
 * Use unittest.skipUnless(os.name == "nt", ...) on test_GetLastError()

Lib/ctypes/test/test_errno.py

@@ -1,12 +1,17 @@
 import unittest, os, errno
 from ctypes import *
 from ctypes.util import find_library
+from test import test_support
+try:
     import threading
+except ImportError:
+    threading = None
+
+libc_name = find_library("c")
 
 class Test(unittest.TestCase):
+    @unittest.skipUnless(libc_name, 'Unable to find the C library')
     def test_open(self):
-        libc_name = find_library("c")
-        if libc_name is not None:
         libc = CDLL(libc_name, use_errno=True)
         if os.name == "nt":
             libc_open = libc._open
@@ -21,6 +26,10 @@ class Test(unittest.TestCase):
         self.assertEqual(set_errno(32), errno.ENOENT)
         self.assertEqual(get_errno(), 32)
 
+    @unittest.skipUnless(libc_name, 'Unable to find the C library')
+    @unittest.skipUnless(threading, 'This test requires threading.')
+    def test_open_thread(self):
+        self.assertEqual(set_errno(32), errno.ENOENT)
 
         def _worker():
             set_errno(0)
@@ -41,8 +50,7 @@ class Test(unittest.TestCase):
         self.assertEqual(get_errno(), 32)
         set_errno(0)
 
-    if os.name == "nt":
-
+    @unittest.skipUnless(os.name == "nt", 'Test specific to Windows')
     def test_GetLastError(self):
         dll = WinDLL("kernel32", use_last_error=True)
         GetModuleHandle = dll.GetModuleHandleA