bpo-31173: Rewrite WSTOPSIG test of test_subprocess (#3055)

The current test_child_terminated_in_stopped_state() function test
creates a child process which calls ptrace(PTRACE_TRACEME, 0, 0) and
then crash (SIGSEGV). The problem is that calling os.waitpid() in the
parent process is not enough to close the process: the child process
remains alive and so the unit test leaks a child process in a
strange state. Closing the child process requires non-trivial code,
maybe platform specific.

Remove 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.