[RFC] Only run a subset of testcases in separated processes

This is quite a crude implementation that works, so RFC.
Also, in thinking about this, these options should be reversed - `--with-process-isolation` should maintain the original behavior of running everything in isolation, and a new option should exist to enable the decorator to run only specific tests in isolation. That way we're not changing the behavior for existing users who upgrade versions and just expect it to continue to work the same.