Since 2013, the Python test suite evolved a lot. First, we fixed hundreds of bugs where tests leaked various kinds of resources: processes, threads, file descriptors, memory, etc. Second, python3 -m test -j0 now runs each test file in a subprocess. Maybe we should emit a warning when tests are run sequentially, without -j0. Maybe it should be an opt-in option, rather than the default. But I prefer to close this old issue. Please open a new issue if you want to make further changes in regrtest.