Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/properties/kt2786.kt

@@ -0,0 +1,13 @@
+trait FooTrait {
+        val propertyTest: String
+}
+
+class FooDelegate: FooTrait {
+        override val propertyTest: String = "OK"
+}
+
+class DelegateTest(): FooTrait by FooDelegate() {
+  fun test() = propertyTest
+}
+
+fun box()  = DelegateTest().test()