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/kt2892.kt

@@ -0,0 +1,16 @@
+open class A
+class B : A() {
+    fun foo() = 1
+}
+
+class Test {
+    val a : A = B()
+    private val b : B get() = a as B //'private' is important here
+
+    fun outer() : Int {
+        fun inner() : Int = b.foo() //'no such field error' here
+        return inner()
+    }
+}
+
+fun box() = if (Test().outer() == 1) "OK" else "fail"