Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/enum/abstractMethodInEnum.kt

@@ -0,0 +1,9 @@
+enum class A() {
+    ENTRY: A(){ override fun t() = "OK"}
+
+    abstract fun t(): String
+}
+
+fun f(a: A) = a.t()
+
+fun box()= f(A.ENTRY)