Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/objects/kt1600.kt

@@ -0,0 +1,8 @@
+abstract class Foo<T> {
+    fun hello(id: T) = "O$id"
+}
+
+class Bar: Foo<String>() {
+}
+
+fun box() = Bar().hello("K")