[2.7] bpo-33760: Fix file leaks in test_io. (GH-7361). (GH-7373)

(cherry picked from commit e36837cb)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_io.py

@@ -1091,6 +1091,7 @@ class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     def test_garbage_collection(self):
         # C BufferedReader objects are collected.
         # The Python version has __del__, so it ends into gc.garbage instead
+        self.addCleanup(support.unlink, support.TESTFN)
         rawio = self.FileIO(support.TESTFN, "w+b")
         f = self.tp(rawio)
         f.f = f
@@ -1292,6 +1293,7 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
 
     def test_truncate(self):
         # Truncate implicitly flushes the buffer.
+        self.addCleanup(support.unlink, support.TESTFN)
         with self.open(support.TESTFN, self.write_mode, buffering=0) as raw:
             bufio = self.tp(raw, 8)
             bufio.write(b"abcdef")
@@ -1398,6 +1400,7 @@ class CBufferedWriterTest(BufferedWriterTest, SizeofTest):
         # C BufferedWriter objects are collected, and collecting them flushes
         # all data to disk.
         # The Python version has __del__, so it ends into gc.garbage instead
+        self.addCleanup(support.unlink, support.TESTFN)
         rawio = self.FileIO(support.TESTFN, "w+b")
         f = self.tp(rawio)
         f.write(b"123xxx")