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

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

@@ -0,0 +1,22 @@
+package foo
+
+interface Base {
+    abstract fun foo(x: String): String
+}
+
+class BaseImpl(val s: String) : Base {
+    override fun foo(x: String): String = "Base: ${s}:${x}"
+}
+
+var global = true
+
+class Derived() : Base by if (global) BaseImpl("then") else BaseImpl("else")
+
+fun box(): String {
+    assertEquals("Base: then:!!", Derived().foo("!!"), "delegation by if expression")
+    global = false
+    assertEquals("Base: else:!!", Derived().foo("!!"), "delegation by if expression")
+
+    return "OK"
+}
+