Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/classes/classObjectField.kt

@@ -0,0 +1,7 @@
+class A() {
+    class object {
+        val value = 10
+    }
+}
+
+fun box() = if (A.value == 10) "OK" else "Fail ${A.value}"