Improvements for running dtests

We currently hardcode a requirement that you need at least 27gb of memory to run the resource intensive tests. This is rather annoying as there isn't really a strict hardware requirement and tests can run on smaller machines in a lot of cases (especially if you mess around with HEAP).
We've already got the command line argument --force-resource-intensive-tests, we don't need additional restrictions in place to stop people who shouldn't be running the tests from running them.

We also don't have a way to run only the resource-intensive dtests or only the upgrade tests