I am able to reproduce it on my Mac and as below the values of os.getpid() are the same with the command. I will try to get to a Linux box to see if I can reproduce this. I was always under the assumption that -j 4 runs tests in parallel as it even says in the message "run tests in parallel using n child processes" . Maybe I am stumbling upon a mac os specific issue or understanding something wrong about the regrtest framework. I am using Macbook pro with Mojave.

./python.exe -Wall -m test -R 3:3 -j 20 test_mailbox -m test_clean -F
0:00:00 load avg: 2.31 Run tests in parallel using 20 child processes
0:00:03 load avg: 2.85 [  1/1] test_mailbox failed
os.getpid() = 37760
support.TESTFN = '@test_37760_tmp'
os.getpid() = 37760
support.TESTFN = '@test_37760_tmp'
beginning 6 repetitions
123456
.test test_mailbox failed -- Traceback (most recent call last):
  File "/Users/kasingar/stuff/python/cpython/Lib/test/test_mailbox.py", line 743, in test_clean
    self.assertFalse(os.path.exists(foo_path))
AssertionError: True is not false
Kill <TestWorkerProcess #2 running test=test_mailbox pid=37761 time=3.4 sec> process group
Kill <TestWorkerProcess #3 running test=test_mailbox pid=37763 time=3.4 sec> process group
Kill <TestWorkerProcess #4 running test=test_mailbox pid=37762 time=3.4 sec> process group
Kill <TestWorkerProcess #5 running test=test_mailbox pid=37764 time=3.4 sec> process group
Kill <TestWorkerProcess #6 running test=test_mailbox pid=37765 time=3.4 sec> process group
Kill <TestWorkerProcess #7 running test=test_mailbox pid=37766 time=3.4 sec> process group
Kill <TestWorkerProcess #8 running test=test_mailbox pid=37767 time=3.4 sec> process group
Kill <TestWorkerProcess #9 running test=test_mailbox pid=37768 time=3.3 sec> process group
Kill <TestWorkerProcess #10 running test=test_mailbox pid=37769 time=3.3 sec> process group
Kill <TestWorkerProcess #11 running test=test_mailbox pid=37770 time=3.3 sec> process group
Kill <TestWorkerProcess #12 running test=test_mailbox pid=37771 time=3.3 sec> process group
Kill <TestWorkerProcess #13 running test=test_mailbox pid=37772 time=3.2 sec> process group
Kill <TestWorkerProcess #14 running test=test_mailbox pid=37773 time=3.2 sec> process group
Kill <TestWorkerProcess #15 running test=test_mailbox pid=37774 time=3.2 sec> process group
Kill <TestWorkerProcess #16 running test=test_mailbox pid=37775 time=3.2 sec> process group
Kill <TestWorkerProcess #17 running test=test_mailbox pid=37776 time=3.2 sec> process group
Kill <TestWorkerProcess #18 running test=test_mailbox pid=37777 time=3.2 sec> process group
Kill <TestWorkerProcess #19 running test=test_mailbox pid=37778 time=3.2 sec> process group
Kill <TestWorkerProcess #20 running test=test_mailbox pid=37779 time=3.2 sec> process group

== Tests result: FAILURE ==

1 test failed:
    test_mailbox

Total duration: 3.4 sec
Tests result: FAILURE