add: tests for zero-parser.

sysy_sets/23_if_test4.sy

@@ -0,0 +1,18 @@
+// test if-{if-else}
+int if_ifElse_() {
+  int a;
+  a = 5;
+  int b;
+  b = 10;
+  if(a == 5){
+    if (b == 10) 
+      a = 25;
+    else 
+      a = a + 15;
+  }
+  return (a);
+}
+
+int main(){
+  return (if_ifElse_());
+}