Add another test for string

Author: shulhi

jscomp/test/VariantCoercion.js

@@ -125,4 +125,8 @@ module CoerceVariantBinaryOp = {
   type flag = | @as(0) A | @as(2) B
 
   let x = 0->lor((B :> int))
+
+  type flagStr = | @as("one") One | @as("two") Two
+
+  let y = (One :> string)->String.length
 }