JS: add tests to show that #KT-12865 fixed as well

compiler/testData/codegen/box/classes/implementComparableInSubclass.kt

@@ -0,0 +1,22 @@
+// See KT-12865
+
+package foo
+
+abstract class Base {
+    val x = 23
+}
+
+class Derived : Base(), Comparable<Derived> {
+    val y = 42
+    override fun compareTo(other: Derived): Int {
+        throw UnsupportedOperationException("not implemented")
+    }
+}
+
+fun box(): String {
+    val b = Derived()
+    if (b.x != 23) return "fail1: ${b.x}"
+    if (b.y != 42) return "fail2: ${b.y}"
+
+    return "OK"
+}