add test for dynamic module import

Author: mununki

jscomp/test/Import.js

@@ -46,7 +46,10 @@ eachIntAsync({
 
 var beltAsModule = import("../../lib/js/belt_List.js");
 
+var M;
+
 exports.eachIntAsync = eachIntAsync;
 exports.eachIntLazy = eachIntLazy;
 exports.beltAsModule = beltAsModule;
+exports.M = M;
 /*  Not a pure module */

jscomp/test/Import.res

@@ -10,3 +10,5 @@ let _ = list{1, 2, 3}->eachIntAsync(n => Js.log2("async", n))
 
 module type BeltList = module type of Belt.List
 let beltAsModule = Js.import(module(Belt.List: BeltList))
+
+module M = @res.await Belt.List