[JS IR BE] Fix and refactor interop tests

js/js.translator/testData/box/expression/cast/explicitUpcast.js

@@ -1,3 +1,3 @@
 function createWrongObject() {
-    return new (JS_TESTS.foo.C);
+    return new (JS_TESTS.C);
 }

js/js.translator/testData/box/expression/cast/explicitUpcast.kt

@@ -1,7 +1,6 @@
-// IGNORE_BACKEND: JS_IR
+// KJS_WITH_FULL_RUNTIME
 // SKIP_MINIFICATION
 // This test assumes that external JS code calls Kotlin code directly
-package foo
 
 open class A