Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/closures/simplestClosureAndBoxing.kt

@@ -0,0 +1,7 @@
+fun box() : String {
+    return if (int_invoker( { 7 } ) == 7) "OK" else "fail"
+}
+
+fun int_invoker(gen :  () -> Int) : Int {
+    return gen()
+}