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/exceptionConstructor.kt

@@ -0,0 +1,7 @@
+class GameError(msg: String): Exception(msg) {
+}
+
+fun box(): String {
+  val e = GameError("foo")
+  return if (e.getMessage() == "foo") "OK" else "fail"
+}