How do you test the java code? Unit testing? What percentage of the test coverage of code? Yes, I'll tell you - 0. There is not any test from a professional point of view, and what do you ask?
The best testers is a community that you have now.
Yes, we do have a big community of people that *could* test the new modpack... *HOWEVER*.... At the moment, they are still working on "low hanging fruit" - easily found, easily repeatable bugs... If there were... say, 100,000 people with the test version, probably 90,000 would hit the bug, 30,000 of them would ignore it if possible, or just go to another pack, 20,000 of them would come to the forums/helpdesk and say nothing more than "Y U SUCK AND PUT OUT BROKEN PACK?", another 20,000 would come saying only "Y MY MINECRAFT CRASH?", 15,000 would come with the same exact crashlog, reposting it even though the previous 100 posts were people with the same problem, and the last 5,000 would be ripping out their hair trying to help the other 85,000...
So which one are you?