JS: create new common directory for all generated tests, migrate several tests there

js/js.translator/testData/box/extensionFunction/extensionForSuperclass.kt

@@ -0,0 +1,18 @@
+package foo
+
+open class A() {
+    open fun c() = 2
+}
+
+class B() : A() {
+    override fun c() = 3
+}
+
+fun B.t() = d() + 1
+
+fun A.d() = c() + 3
+
+fun box(): String {
+    if (A().d() == 5 && B().d() == 6 && B().t() == 7) return "OK"
+    return "fail"
+}