Problem:	Text flickers if we always skip update_topline() in
curs_columns() for 'nosplitscroll'.
Solution:	Only skip update_topline() while executing
win_fix_scroll()(Fix vim#11158).

Problem:	Text is scrolled for 'nosplitscroll' when changing
tab page.
Solution:	Skip win_fix_scroll() call when changing tab page
(Fix vim#11159).

Problem:	Text scrolls for 'nosplitscroll' in terminal callback
because State is not updated yet.
Solution:	Treat terminal mode as normal mode in win_fix_cursor()
(Fix vim#11160).

Problem:	'nosplitscroll' tests are outdated.
Solution:	Add tests for various fixes, fix 'scrolloff' not being
tested.

Problem:	Cursor position changes in insert mode with
'nosplitscroll' and 'scrolloff' in a small window.
Solution	Replace previously erroneous workaround by
validate_botline win() call in win_fix_cursor().