Where is OpenJDK?
If you are using Linux as developing OS, this’s not a big deal. You always get OpenJDK from the package. But what if you are using Win or macOS, you won’t get built in OpenJDK. If you are doing some search on the Internet, you may find out this unofficial built for each OS. However, looking carefully what README.md they wrote, they don’t respond any question about the TCK which is like JDK’s unit testing. So no one would know did they test their built. Do you want to use software that you don’t know test or not? I don’t have that courage to use it.
Where is OpenJDK for Win and macOS?
In the end
You could safely use OpenJDK from Azul System and RedHat which will be tested before it releases.
Here lists companies have TCK could verify each version of JDK.