This is more of an observation than an answer, but it may help others who were as frustrated as I was.
I kept getting this error from two tests in my suite. I thought I had simply broken the tests with the refactoring I was doing, so after backing out changes didn't work, I reverted to earlier code, twice (two revisions back) thinking it'd get rid of the error. Doing so changed nothing. I chased my tail all day yesterday, and part of this morning without resolving the issue.
I got frustrated and checked out the code onto a laptop this morning. Ran the entire test suite (about 180 tests), no errors. So the errors were never in the code or tests. Went back to my dev box and rebooted it to clear anything in memory that might have been causing the issue. No change, same errors on the same two tests. So I deleted the directory from my machine, and checked it back out. Voila! No errors.
No idea what caused it, or how to fix it, but deleting the working directory and checking it back out fixed whatever it was.
Hope this helps someone.