JS: add test to prove that KT-11910 is not reproducible anymore

compiler/testData/codegen/box/localClasses/subclassingExtensionReceiverClass.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND: JVM
+
+class A(val x: String) {
+    open inner class AB(val y: String) {
+        fun bar() = x + y
+    }
+}
+
+fun A.foo(u: String, v: String, w: String): A.AB {
+    class FooC(z: String) : A.AB("$z$v$w")
+    return FooC(u)
+}
+
+fun box(): String {
+    val r = A("1").foo("2", "3", "4").bar()
+    if (r != "1234") return "fail: $r"
+
+    return "OK"
+}