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: ":set scroll&" often gives an error.
Solution: Don't use a fixed default value, use half the window height. Add a
test. (Ozaki Kiichi, closes#2104)
