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/kt257.kt

@@ -0,0 +1,20 @@
+class A<T>(var t: T) {}
+class B<R>(val r: R) {}
+
+fun box() : String {
+    val ai = A<Int>(1)
+    val aai = A<A<Int>>(ai)
+    if(aai.t.t != 1)  return "fail"
+/*
+    aai.t.t = 2
+    if(aai.t.t != 2)  return "fail"
+
+    if(ai.t != 2)  return "fail"
+    if(aai.t != ai)  return "fail"
+    if(aai.t !== ai) return "fail"
+
+    val abi = A<B<Int>>(B<Int>(1))
+    if(abi.t.r != 1) return "fail"
+*/
+    return "OK"
+}