Urgh. I do (business) software development for a living, and I understand the exact problems PDX faces here. Honestly, I'm as frustrated as the next fan by the fact that apparently every Ascension path (haven't tried Psionics yet) are apparently broken, as are important event chains like the Worm (no Omega Theory), so I've had to resort to the console to just make my game *work* and prevent game-ending brokenness.
But the fact is that the only way to find the kind of bugs that have slipped into 1.5 is to do full-scale integration testing - in terms of a game that means playing full games to completion, and in terms of Stellaris that means 20+ hours on each test. This is prohibitive, and you'll need to redo tests regularly as other (seemingly unrelated) code changes can suddenly break these features again. In my smaller scale business projects I can reasonably automate 95% of testing with proper unittests and integration tests. Not so for a game - and particularly not a game like Stellaris.
Don't mix terms. What you're talking about is called regression tests and continuous integration. Integration testing is only a very small part of testing. And unit testing itself is only a tiny part of both those features. I also have to disagree with your statement that you can't automate most of Stellaris' testing. Anything that is not UI can be unit tested, period. And even UI can be unit tested if you have the proper frameworks for it (although the tests' outcome might not be as helpful in this case, depending). You don't have to play the game to test functionality. Beta players are there to catch bugs not yet caught by tests, and test user experience and joy-of-use/UI design.
I actually do doubt whether Paradox has proper regression testing and continuous integration facilities. In fact, many small-to-middle IT companies still lack proper facilities for that and I have only seldomly heard of CI/RT in gaming.
That being said, I think it would help Paradox and the community a lot, if they would start adding a public interface for their bug-tracking system, where they can filter what the users are allowed to see and what not (as Paradox employees have explained, there are bugs that are not to be disclosed to the public).