Not unit tests, but a fair bit of in-line verification that the correct base has been added. There are numerous DNA proofreading mechanisms to prevent errors. If they didn't exist, we would be riddled with mutations.
More specifically, it's DNA-Polymerase who proofreads. You should also check out the T-Cells (innate and acquired immunity) work. It blew my mind the level of complexity, diversification, etc: There are cells that perform profiling, cells the memorize information about viruses (that's how vaccines work) already encountered, etc.
Molecular biology is like entering the rabbit whole.
Imagine a wormhole opening up in front of you and a computer from the unimaginably distant future drops out in front of you.
You very likely wouldn't know it's even a computer. Countless billions of hours a futures mankinds time has been poured into it shrinking each component to the atomic scale. Generation after generation of hardening against future hackers attacks, and even self aware viruses have increased the machines complexity beyond human understanding. The code the computer interprets would be a lost language from a civilization that doesn't exist yet.
And that is pretty much where we are in understanding the interworkings of DNA today. Nature has a 3 billion year jump on us, and it's going to take us a long time to unravel it all.
