Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/arrays/longAsIndex.kt

@@ -0,0 +1,9 @@
+fun IntArray.set(index: Long, elem: Int) { this[index.toInt()] = elem }
+fun IntArray.get(index: Long) = this[index.toInt()]
+
+fun box(): String {
+    var l = IntArray(1)
+    l[0.toLong()] = 4
+    l[0.toLong()] += 6
+    return if (l[0.toLong()] == 10) "OK" else "Fail"
+}