Add 4 unit tests covering the new W50 editor commands:
- mark_all_selects_entire_buffer: verify selection covers
the full buffer (start=0, end=buffer.len())
- close_clean_buffer_reopens_empty: verify Close on an
unmodified buffer resets to an empty buffer
- block_save_on_no_selection_shows_message: verify the
"No selection" message when nothing is selected
- insert_file_opens_insert_file_prompt: verify the mode
switches to InsertFile prompt
Tests: 1415 pass (was 1411), zero warnings.
vasilito
69dff0c26c