I think what you are asking is: 'given this code fails with an exception, how can I assure that the test itself fails?' That's a great thing to do, IMHO. I think there are two ways.
1) don't catch the exception; instead declare your AdaugProdusLaCos() method with a "throws Exception" clause.
2) in your catch block, if you want to be explicit about the failure, you can add "Assert.fail(ex.getMessage());", or whatever else you want to say.
This second one will let you be very exact. But you will have to import the Assert class. That's under org.testng.Assert. Here's a page that includes this info.
Good luck, and thanks for indirectly introducing me to this. In the past I had worked with HtmlTest, HttpTest and even one I wrote myself a long time ago. This "testng" or "Selenium" looks quite convenient and very similar to Junit.
If a chicken that is half full crosses the road, will anyone hear it?