> I was always under the assumption that -j 4 runs tests in parallel

If you run one test file, even with -j1000, only one test file will be run in parallel :-)

To run the same file in parallel, you have to ask to run it more than once: "-j2 test_mailbox test_mailbox" or "-j2 test_mailbox -F" for example.

-F runs the same tests in a loop until it fails. It's a convenient way to run the same test in parallel to detect race conditions ;-)