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/ko_test.c

@@ -3,22 +3,10 @@
 static Str
 kotrans(char *keys)
 {
-	Emit e;
-	Im state;
-	Str out;
-	char *p;
+	Str out, raw;
 
-	memset(&state, 0, sizeof state);
-	state.l = getlang(LangKO);
-	sclear(&out);
-	for(p = keys; *p != '\0'; p++){
-		e = transko(&state, (uchar)*p);
-		sappend(&out, &e.s);
-		state.pre = e.next;
-		if(!e.eat)
-			sputr(&out, (uchar)*p);
-	}
-	sappend(&out, &state.pre);
+	raw = mkstr(keys);
+	transstr(getlang(LangKO), &raw, &out);
 	return out;
 }