Fix slk test and check files due to libcurses slk changes.

New tests and updates for corrected behaviour due to fix for PR56224
* Added extra testing to the addstr test to demonstrate bug described
in PR#56224 and validate case when scrolling enabled still works.
* Fixed slk test, the slk_init output changed due to corrected wrapping,
slk_restore no longer returns ERR probably due to addwchar no longer
returning ERR when an implicit scroll was attempted when scrolling
disabled. Commented out the slk_wset test, this is now returning ERR
instead of misbehaving, needs investigation.

branches: 1.1.4;
Merge in code from Google Summer of Code project which dramatically
increases the number of tests performed.

Thanks to Naman Jain <jnaman806@gmail.com> for his excellent work on
this GSoC project.