wc: Improve test coverage.

[des]

Summary

• Add a test that causes mbrtowc() to return 0.

• Add a test that causes mbrtowc() to return -2.

Sponsred by: Klara, Inc.

[jlduran_gmail.com]

Elegant test implementation! I wonder if the Freudians will find it too scatological.

[emaste]

usr.bin/wc/tests/wc_test.sh

164

maybe worth a comment demonstrating how the mb char crosses the boundary (to aid in someone updating this if the buffer increases in the future)?

[des]

Parametrize MAXBSIZE, make sure we really do cross a buffer boundary, and explain what we're doing and why.