Lower call stack size for test_big_recursive_cobegin and ManyThreads

Both of these tests fail on some of our machines that have ~4GB of memory. Both create a lot of tasks which means a lot of stacks. With qthreads these tasks are being created faster than they were with fifo which means they might use more memory at any given moment (same amount of memory used total though.) In general I've been trying to lower correctness tests to use at most 4GB of ram. 

Even though there are a lot of tasks I think we can leave guard pages on, as they don't seem to have any real performance impact for these two (both tests finish in fractions of a second.)