Problem: ":z!" is not supported.
Solution: Make ":z!" work and add tests. (Dominique Pellé, closes#8836)
Use display height instead of current window height.
Problem: Various inconsistencies in test files.
Solution: Add modelines where they were missing. Use Check commands instead
of silently skipping over tests. Adjust indents and comments.
(Ken Takata, closes#6695)
Problem: The cursor line number becomes negative when using :z^ in an empty
buffer. (neovim #6557)
Solution: Correct the line number. Also reset the column.
