Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/classes/kt725.kt

@@ -0,0 +1,8 @@
+fun Int?.inc() = this!!.inc()
+
+public fun box() : String {
+    var i : Int? = 10
+    val j = i++
+
+    return if(j==10 && 11 == i) "OK" else "fail"
+}