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.

fn.h

@@ -29,6 +29,8 @@ int	pagemarker(char*, int, int, int, int);
 void	popuplayout(Drawcmd*, int, int, Popup*);
 void	popupdraw(u32int*, Drawcmd*, Popup*);
 void	imthread(void*);
+void	impre(Im*, Str*);
+void	transstr(Lang*, Str*, Str*);
 Emit	transmap(Im*, Rune);
 Emit	transko(Im*, Rune);
 Emit	transvi(Im*, Rune);