Make all the invalid fd tests for os subject to the function being available.

Lib/test/test_os.py

@@ -547,12 +547,15 @@ class TestInvalidFD(unittest.TestCase):
         locals()["test_"+f] = get_single(f)
 
     def test_isatty(self):
+        if hasattr(os, "isatty"):
             self.assertEqual(os.isatty(10), False)
 
     def test_closerange(self):
+        if hasattr(os, "closerange"):
             self.assertEqual(os.closerange(10, 20), None)
 
     def test_dup2(self):
+        if hasattr(os, "dup2"):
             self.assertRaises(OSError, os.dup2, 10, 20)
 
     def test_fchmod(self):
@@ -573,9 +576,11 @@ class TestInvalidFD(unittest.TestCase):
             self.assertRaises(IOError, os.ftruncate, 10, 0)
 
     def test_lseek(self):
+        if hasattr(os, "lseek"):
             self.assertRaises(OSError, os.lseek, 10, 0, 0)
 
     def test_read(self):
+        if hasattr(os, "read"):
             self.assertRaises(OSError, os.read, 10, 1)
 
     def test_tcsetpgrpt(self):
@@ -583,6 +588,7 @@ class TestInvalidFD(unittest.TestCase):
             self.assertRaises(OSError, os.tcsetpgrp, 10, 0)
 
     def test_write(self):
+        if hasattr(os, "write"):
             self.assertRaises(OSError, os.write, 10, " ")
 
 if sys.platform != 'win32':