Fair enough, though interactions with other processes and network conditions are not relevant for unit tests. Timings maybe, but then you still need a strategy to turn a failing test into information you can use to fix a bug.
I have horrible memories of time spent tracking down other test failures caused by helper programs not shutting down cleanly, and causing obscure bugs in later tests. e.g. a test failing because it couldn’t bind to port 443, because an earlier test helper program sometimes didn’t clean up properly. That’s just one possibility for a process and/or network condition that ruins a test.