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/iteratorByteArrayNextByte.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val a = ByteArray(5)
+    val x = a.iterator()
+    var i = 0
+    while (x.hasNext()) {
+        if (a[i] != x.nextByte()) return "Fail $i"
+        i++
+    }
+    return "OK"
+}