Add option to use overflow: scroll in text fixtures

Summary: Wires up the option to use overflow: scroll in JS, Java, and CPP gentest fixtures.

Reviewed By: NickGerleman

Differential Revision: D50225803

fbshipit-source-id: f71c000b7143ad9e0e34d99e2d5ba2c6186266d1

gentest/gentest.js

@@ -520,6 +520,8 @@ function overflowValue(e, value) {
       return e.YGOverflowVisible;
     case 'hidden':
       return e.YGOverflowHidden;
+    case 'scroll':
+      return e.YGOverflowScroll;
   }
 }