where i worked and supervised on the midnight shift [42+ years] we water tested many of our products.
the lowest pressure vessels were tested to 1 1/2 times their working pressure, except for some specialty items that were tested to 2 times working pressure.
our high pressure vessels were tested at three times working pressure, except for a few items that were tested up to five times working pressure, depending on application.
these products were CLEARLY marked as to the working pressure to be NOT exceeded.
however, heater cores in my opinion, should be tested at 25# [roughly two times cap pressure of 13#]. this is on the condition the outside appearance looks VERY GOOD. if it looks bad on the outside, i consider the internal condition to be the same or worse. if that's the case, a re-core or replacement is about the only choice you have.
remember, this is only MY opinion, your results will vary.