Fuck a porn star? There are few things less appealing than fucking a porn star. Not without a ten foot pole anyway...
Yes, porn stars do get tested for STDs regularly which I believe is generally twice a month. What about the period in between tests? You don't know who they just fucked yesterday or earlier that day.
You must realize for every porn star who has had a long career there are 50 who lasted a minute. There are reasons for that... Yes, many just can't hack it but drug use and STDs are high on the list.
Porn stars are nasty... Period.