Run the Python test suite in parallel by default #159155

Currently, python -m test runs tests sequentially in a single process by default. It's slow and unsafe: if a test does crash, the whole test suite is interrupted, we don't exit cleanly, and resources may not be released properly (a temporary directory can be left behind).

I propose to run the Python test suite in parallel by default: choose the number of worker processes using os.process_cpu_count(). In short, make -j0 option the default. Running tests in parallel makes the task way faster (especially on machines with 8 CPUs or more). When using -j0, the test suite main process just watchs worker processes and is way likely to crash. When a worker process does crash, its working directory is removed cleanly. Also, when tests are interrupted by CTRL+C, the main process makes sure that all worker processes are stopped or killed (including child processes).

Instead, you will have to use --single-process explicitly to opt-in for Python 3.15 behavior: run tests sequentially in a single process.