Test Parallelism in Russ

Russ will run all of your tests in parallel. If you change the global allocator, that would be true for all of the tests that are running in parallel. So I think what I'd have to do is basically give up on test parallelism if I want to do that. The only other theory that I had was doing something along the lines of like at the beginning of each test trying to like set a thread local flag and then maybe that'll work. But I don't know, I'm not like super optimistic.