How can I prevent AppDomainUnloadedException after NUnit tests PLINQ code?

Question

How can I diagnose and minimize or prevent AppDomainUnloadedException?

NUnit 2.5.2 consistently throws AppDomainUnloadedException after long (>10s) tests involving PLINQ.

Back in July 2008, Stephen Toub said:

Yes, the scheduler in the CTP doesn't handle thread aborts very well, which frequently causes the process to crash when a live scheduler exists in a domain being shutdown (since AppDomain shut down causes thread aborts on all threads with stack frames in that domain). We're working to harden this for a future release.

I've tried many workarounds, including:

• Running the test in a separate method to eliminate stray references
• Specifying /domain:None as an NUNit argument
• Removing the legacyUnhandledAppDomainPolicy element from nunit-console.exe.config

I can't backgrade NUnit to reduce the odds of the race condition, as I need PLINQ to make my parametric tests go faster. Versions of NUnit without the problem don't support parametric tests.