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

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

@@ -0,0 +1,33 @@
+package foo
+
+interface Base {
+    var prop: String
+    var Int.foo: String
+}
+
+open class BaseImpl(val s: String) : Base {
+    override var prop: String = "init"
+    override var Int.foo: String
+        get() = "get Int.foo:${s}:${this}"
+        set(value) {
+            prop = "set Int.foo:${s}:${this}:${value}"
+        }
+
+}
+
+class Derived() : Base by BaseImpl("test") {
+    fun getFooValue(x: Int): String = x.foo
+    fun setFooValue(x: Int, value: String) {
+        x.foo = value
+    }
+}
+
+fun box(): String {
+    var d = Derived()
+    assertEquals("get Int.foo:test:5", d.getFooValue(5))
+
+    d.setFooValue(10, "A")
+    assertEquals("set Int.foo:test:10:A", d.prop)
+
+    return "OK"
+}