I'm the chief cat herder over on AdoptOpenJDK. It is a build farm in progress with the end goal of providing OpenJDK LTS support (matching Oracle's timelines) for OpenJDK on all platforms that Java currently runs on (including ARM32/64, zos, AIX, z390 as well as Linux x86, Mac OS X, windows etc). Wee'll also support OpenJDK variants such as Eclipse OpenJ9, SAP OpenJDK, OpenJFX, and some of the early builds of experimental new features.
It's also an infrastructure as code project so OpenJDK vendors have a common place to shared build technology and workflows. For example we currently have IBM, Red Hat, Oracle, SAP and other vendor engineers working on this.
The release builds will be TCK tested, as well as a variety of other common industry test suites. The quality level will be as high if not higher than existing vendors today.
I appreciate there's probably a lot of confusion (especially as this is still a work in progress) so please fire questions my way!