Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/namespace/incrementProperty.kt

@@ -0,0 +1,14 @@
+class Slot() {
+  var vitality: Int = 10000
+
+  fun increaseVitality(delta: Int) {
+    vitality += delta
+    if (vitality > 65535) vitality = 65535;
+  }
+}
+
+fun box(): String {
+  val s = Slot()
+  s.increaseVitality(1000)
+  return if (s.vitality == 11000) "OK" else "fail"
+}