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/kt1759.kt

@@ -0,0 +1,11 @@
+class Greeter(var name : String) {
+    fun greet() {
+        name = name.plus("")
+        System.out?.println("Hello, $name");
+    }
+}
+
+fun box() : String {
+    Greeter("OK").greet()
+    return "OK"
+}