If your questioning whether a particular bolt is safe or not, the only appropriate answer is to replace the bolt. And no 15 kN is not strong enough, that's why UIAA requires 25 kN for sheer. Its very possible to produce 15 kN on the top anchor on a fall straight into the belay station.
A tester is usually used for testing bolts not breaking bolts.
You don't want to pull a bolt to anywhere near its breaking strength because it weakens them.
15kn is more than strong enough for a bolt. Your body will break before the bolt pulls.
We usually test to 7.5kn.