Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/when/whenArgumentIsEvaluatedOnlyOnce.kt

@@ -0,0 +1,13 @@
+var x = 0
+fun inc(): Int {
+    x++
+    return 0
+}
+fun box(): String {
+  val al = java.util.ArrayList<Int>()
+  when (inc()) {
+      in al -> return "fail 1"
+      else -> {}
+  }
+  return if (x == 1) "OK" else "fail 2"
+}