https://github.com/python/cpython/pull/3055 removes the functional test and replaces it with an unit test which mocks os.waitpid() using a new _testcapi.W_STOPCODE() function to test the WIFSTOPPED() path.

The functional test created a core dump, but it's now fixed using SuppressCrashReport. It leaks a zombie process in a special state, the process is traced and cannot be killed. I tried to wait for the process a second time, but it's not enough to "close" it. I guess that we would have to write a little debugger to attach the process in the parent process. IMHO it's overcomplicated just to check that subprocess calls WIFSTOPPED().