KT-12877: modify tests to check whether mutable top-level properties of JsModule files are supported

js/js.translator/testData/box/jsModule/externalPackage.kt

@@ -16,6 +16,8 @@ package foo
 
 @native val bar: Int = noImpl
 
+@native var mbar: Int = noImpl
+
 fun box(): String {
     val a = A(23)
     assertEquals(23, a.x)
@@ -27,5 +29,8 @@ fun box(): String {
     assertEquals(365, foo(42))
     assertEquals(423, bar)
 
+    mbar = 523
+    assertEquals(523, mbar)
+
     return "OK"
 }