bpo-31904: skip some tests related to fifo on VxWorks (GH-23473)

On VxWork RTOS, FIFO must be created under directory "/fifos/". Some test cases related to fifo is invalid on VxWorks. So skip them.

Lib/test/test_pathlib.py

@@ -2219,6 +2219,8 @@ class _BasePathTest(object):
         self.assertIs((P / 'fileA\x00').is_fifo(), False)
 
     @unittest.skipUnless(hasattr(os, "mkfifo"), "os.mkfifo() required")
+    @unittest.skipIf(sys.platform == "vxworks",
+                    "fifo requires special path on VxWorks")
     def test_is_fifo_true(self):
         P = self.cls(BASE, 'myfifo')
         try: