I recently upgraded from 7.1 to 8.0 which when I noticed this problem.
While trying to run all 540 of my project's unit tests I noticed that the unit test runner was chewing up ever increasing amounts of my computer's memory, going almost to 100% Physical Memory usage. The runner itself was using ~2.6 GB of memory, and yet was pretty much stuck. I've closed and reopened Visual Studio, I've rebooted my computer, I've run only some of the tests at a time and yet the problem still occurs. Before I upgraded the tests would run just fine, without eating up all of my memory.