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/forInSmartCastedToArray.kt

@@ -0,0 +1,10 @@
+fun f(x: Any?): String {
+    if (x is Array<String>) {
+        for (i in x) {
+            return i
+        }
+    }
+    return "FAIL"
+}
+
+fun box(): String = f(Array<String>(1, {"OK"}))