Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/enum/abstractmethod.kt

@@ -1,10 +0,0 @@
-fun box() = IssueState.DEFAULT.ToString() + IssueState.FIXED.ToString()
-
-enum class IssueState {
-   DEFAULT
-   FIXED {
-       override fun ToString() = "K"
-   }
-
-   open fun ToString() : String = "O"
-}