bpo-18174: Fix file descriptor leaks in tests (GH-7408)

* test_tempfile.test_no_leak_fd() mocks os.close() but it doesn't
  call the original os.close() method and so leaks an open file
  descriptor. Fix the test by calling the original os.close()
  function.
* test_posix.test_fdopen_directory(): close the directory file
  descriptor when the test completes.
diff --git a/Lib/test/test_tempfile.py b/Lib/test/test_tempfile.py
index 5c111a2..2efb836 100644
--- a/Lib/test/test_tempfile.py
+++ b/Lib/test/test_tempfile.py
@@ -821,6 +821,7 @@
         old_fdopen = os.fdopen
         closed = []
         def close(fd):
+            old_close(fd)
             closed.append(fd)
         def fdopen(*args):
             raise ValueError()