test: Cover StaticFileHandler subclass patterns from downstream packages

[bdarnell]

Add tests modeled on how popular downstream packages subclass StaticFileHandler, so that changes to the base class that would break them are caught here:

• A multi-root handler (as in jupyter_server) installed as the static_handler_class with a list static_path, used through static_url; and make_static_url called directly with static_path=None by a handler that finds its own files (voila).
• A handler that routes by path prefix to one of several roots and calls super().get_absolute_path with that root (Bokeh). A ../ from one of its roots into another is rejected by the base get_absolute_path, even though the subclass's own validation would accept it.
• A handler that sets allowed_symlink_directory itself during validate_absolute_path, following symlinked directories (Jupyter's actual FileFindHandler).
• A get_absolute_path instance method that reassigns self.root, which get() must then pass to validate_absolute_path (voila).
• A handler whose path is a single file, possibly a symlink (JupyterHub's LogoHandler).

Claude-Session: https://claude.ai/code/session_019nvEV873ZwBncgxPP13Xfd