Has anyone found good ways to deal with a situation where a senior developer solely responsible for a particular component seems to be "doing a favor" by letting QA test his code, and refuses to invest in making the component more testable, answering questions, etc.?
The person is very senior, and there's no option of letting bugs in that component to go into production just to "teach a lesson."
Is this making your job harder? If it is, talk to his boss about it. Since he clearly has no interest in improving to make the work go more smoothly, someone up the chain of command ought to tell him he's wrong.
Be very careful, though, if this guy is your boss.
How to Win Friends and Influence People
...you can make someone want to do what you want them to by seeing the situation from the other person's point of view and "arousing in the other person an eager want."
It's really a question of manipulation and subtle control.
If he's really arrogant, you could flat out attack it -- "if you're not good enough to produce testable components, that's fine; we'll put you on something else." Just reply to any counter argument with "whatever." For these guys, publicly respect the individual but privately respect their intelligence. Just be sure when you leave, he believes you think he's "weak."
If he's questioning the collective intelligence of QA, challenge him to create something "even they could understand."
If he considers you a peer, just play dumb when reading his code. Keep asking the simple questions until he rolls his eyes and simplifies it for you.