Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/strings/kt889.kt

@@ -0,0 +1,9 @@
+fun Int.plus(s: String) : String {
+  System.out?.println("Int.plus(s: String) called")
+  return s
+}
+
+fun box() : String {
+   val s = "${1 + "a"}"
+   return if(s == "a") "OK" else "fail"
+}