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/accessToPrivateSetter.kt

@@ -0,0 +1,13 @@
+class D {
+    var foo = 1
+        private set
+
+    fun foo() {
+        foo = 2
+    }
+}
+
+fun box(): String {
+   D().foo()
+   return "OK"
+}