[3.15] gh-151693: Make the curses tests portable to other curses implementations...

[3.15] gh-151693: Make the curses tests portable to other curses implementations (GH-151729) (GH-151733)

Make the curses tests portable to other curses implementations

Guard the chgat() check (chgat() needs wchgat()) and stop assuming a
subpad shares the parent pad's cells (implementation-defined in X/Open).
(cherry picked from commit 64fab74bd7288bfa67cd7727452febdaafed4270)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>