The past few days, I've worked on T336630 and T336624, both of which describe failures I was seeing which other people weren't. In both cases, it turned out to be caused by something in my user-config.py. Many of these tests also rely on external resources (i.e. live testing against testwiki or metawiki). I come from a shop where our test environment was hermetic. That eliminated this kind of flakiness due to environmental differences. It's a great way to work, but I recognize that's not what we've got here, and not easy to get to that point.
Given all that, any suggestions on how I should set up a dev environment on my local machine to minimize this kind of problem in the future?
Also, once I figured out what was going on, I closed T336630 as "declined". Was that the right state for "I'm not going to do any more work on this because it turned out to be a problem with the test environment". Should I have used "invalid"? Something else?