Problem: tests: no test for mapping with special keys in session file.
Solution: Add a special keys to an existing test. Also test with UTF-8
characters containing 0x80 or 0x9b bytes (zeertzjq).
closes: vim/vim#173609ff1e598e8
(cherry picked from commit 071dcab68f)
Problem: multi-byte mappings not properly stored in session file
Solution: unescape the mapping before writing out the mapping, prefer
single-byte mapping name if possible (Miguel Barro)
closes: vim/vim#173555b07aff2f6
Co-authored-by: GuyBrush <miguel.barro@live.com>
(cherry picked from commit 153a910897)