Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/controlStructures/kt2147.kt

@@ -0,0 +1,9 @@
+class Foo {
+  fun isOk() = true
+}
+
+fun box(): String {
+   val foo: Foo? = Foo()
+   if (foo?.isOk()!!) return "OK"
+   return "fail"
+}