bpo-37421: Fix test_shutil: don't leak temporary files (GH-14416)

* Fix typo in supports_file2file_sendfile(); ensure that dst is
  removed
* Fix test_copytree_custom_copy_function(): remove dst tree.
  Use support.rmtree() rather than shutil.rmtree() to remove
  temporary directories: support tries harder.
2 files changed