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

js/js.translator/testData/box/delegation/delegationExtFun1.kt

@@ -0,0 +1,19 @@
+package foo
+
+interface Base {
+    abstract fun Int.foo(): String
+}
+
+open class BaseImpl(val s: String) : Base {
+    override fun Int.foo(): String = "Int.foo ${s}:${this}"
+}
+
+class Derived() : Base by BaseImpl("test") {
+    fun bar(x: Int): String = x.foo()
+}
+
+fun box(): String {
+    assertEquals("Int.foo test:5", Derived().bar(5))
+
+    return "OK"
+}