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

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

@@ -17,6 +17,8 @@ package foo
 
 @native val bar: Int = noImpl
 
+@native var mbar: Int = noImpl
+
 // FILE: lib2.kt
 package foo
 
@@ -40,5 +42,8 @@ fun box(): String {
 
     assertEquals(12345, C.f())
 
+    mbar = 523
+    assertEquals(523, mbar)
+
     return "OK"
 }