There have been many instances where the OOM killer has killed a mongod process when running on the UBSan builder. With the num_jobs_available expansion equal to the number of CPUs, we end up running 4 tests concurrently on the ubuntu1604-test distro is a c3.xlarge (4 CPU, 7.5GiB memory). We should change to use the ubuntu1604-build distro and set num_jobs_available to number of CPUs divided by 3 when running tests to match the configuration we use for the ASan builders. The ubuntu1604-build distro is a c3.4xlarge (16 CPU, 30GiB memory), so we'll end up running 5 tests concurrently, but have a larger ratio (3.2x) of available memory to number of concurrent tests.
Note: The special handling for running the "compile" task on the ubuntu1604-build distro can be removed since all tasks for the build variant will run on that distro anyway.