test-systemd-tmpfiles.py: create global temporary directory

test-systemd-tmpfiles.py: create global temporary directory

For some unknown reasons, the temporary directory created by the test
below is not removed:
```
 # Test the case that a valid symlink is in the path.
 label = 'valid_symlink-deep'
 test_content('f= {} - - - - ' + label, label, user=user, subpath='/deep/1/2', path_cb=valid_symlink)
```
To keep /tmp clean, let's create the global temprary directory.