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/vi_test.c

@@ -3,24 +3,10 @@
 static void
 typevi(struct ct *t, char *keys, char *want)
 {
-	Emit e;
-	Im state;
-	Str out, shown;
-	char *p;
+	Str out, raw;
 
-	memset(&state, 0, sizeof state);
-	state.l = &testvi;
-	sclear(&out);
-	for(p = keys; *p != '\0'; p++){
-		e = transvi(&state, (uchar)*p);
-		sappend(&out, &e.s);
-		state.pre = e.next;
-		state.raw = e.raw;
-		if(!e.eat)
-			sputr(&out, (uchar)*p);
-	}
-	shown = shownpre(&state);
-	sappend(&out, &shown);
+	raw = mkstr(keys);
+	transstr(&testvi, &raw, &out);
 	checkstr(t, keys, want, &out);
 }
 
@@ -79,7 +65,7 @@ vietnamese_backspace(struct ct *t)
 		state.raw = mkstr(cases[i].raw);
 		backvi(&state);
 		checkstr(t, cases[i].raw, cases[i].rawafter, &state.raw);
-		shown = shownpre(&state);
+		impre(&state, &shown);
 		checkstr(t, cases[i].raw, cases[i].after, &shown);
 	}
 }