test: drive Korean and Vietnamese through the engine's own transstr

ko_test and vi_test replayed keys with private copies of transstr's
loop, and test_util reimplemented the shown preedit; the engine exports
impre(Im*, Str*) and transstr() instead, so a change to composition
rules is one edit.

tests/test.h

@@ -6,7 +6,6 @@ extern Lang testvi;
 
 Str	mkstr(char*);
 int	checkstr(struct ct*, char*, char*, Str*);
-Str	shownpre(Im*);
 
 void	str_init_utf8(struct ct*);
 void	str_edit_and_alias(struct ct*);