We already tested all corner cases for two-byte and three-byte UTF-8

We already tested all corner cases for two-byte and three-byte UTF-8
sequences, but coverage of four-byte sequences was incomplete, which
contributed to the recently fixed ksh(1) emacs.c bug not being found
for some time.

Consequently, add some tests covering
+ valid sequences starting with \0360, \0361, and \0363;
+ incomplete sequences starting with \0360 and \0361;
+ invalid (too low) sequences starting with \0360.

OK anton@