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

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.