[3.14] Improve tests for the PyUnicodeWriter C API (GH-146157) (GH-146180)

Add tests for corner cases: NULL pointers and out of range values.
(cherry picked from commit ab47892c32e6361f2180e7d86682650f0850c1c4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>