Stop marking test__fileobject.py as allowed to fail. That's hiding the true...

Stop marking test__fileobject.py as allowed to fail. That's hiding the true failure I just introduced in the prior commit---which only seems to affect Python 2.7
[skip travis]

src/greentest/known_failures.py

@@ -62,15 +62,6 @@ if sys.platform == 'win32':
             # This one sometimes randomly closes connections, but no indication
             # of a server crash, only a client side close.
             'FLAKY test__server_pywsgi.py',
-            # We only use FileObjectThread on Win32. Sometimes the
-            # visibility of the 'close' operation, which happens in a
-            # background thread, doesn't make it to the foreground
-            # thread in a timely fashion, leading to 'os.close(4) must
-            # not succeed' in test_del_close. We have the same thing
-            # with flushing and closing in test_newlines. Both of
-            # these are most commonly (only?) observed on Py27/64-bit.
-            # They also appear on 64-bit 3.6 with libuv
-            'FLAKY test__fileobject.py',
         ]
 
         if PYPY and LIBUV:

src/greentest/test__fileobject.py

@@ -12,6 +12,7 @@ import greentest
 from greentest.sysinfo import PY3
 from greentest.flaky import reraiseFlakyTestRaceConditionLibuv
 from greentest.skipping import skipOnLibuvOnCIOnPyPy
+from greentest.skipping import skipOnWindows
 
 try:
     ResourceWarning
@@ -62,10 +63,20 @@ class Test(greentest.TestCase):
         with FileObject(r, 'rb') as fobj:
             self.assertEqual(fobj.read(), b'x')
 
+    # We only use FileObjectThread on Win32. Sometimes the
+    # visibility of the 'close' operation, which happens in a
+    # background thread, doesn't make it to the foreground
+    # thread in a timely fashion, leading to 'os.close(4) must
+    # not succeed' in test_del_close. We have the same thing
+    # with flushing and closing in test_newlines. Both of
+    # these are most commonly (only?) observed on Py27/64-bit.
+    # They also appear on 64-bit 3.6 with libuv
+    @skipOnWindows("Thread race conditions")
     def test_del(self):
         # Close should be true by default
         self._test_del()
 
+    @skipOnWindows("Thread race conditions")
     def test_del_close(self):
         self._test_del(close=True)