Clarify failed parallel tests run message

[Adam Zapletal]

If the test suite fails while running with more than one process, and the traceback is cannot be pickled, this message is printed:

You should re-run this test without the --parallel option to reproduce the failure and get a correct traceback.

The CPU count on my machine is 4, and the test suite runs with 4 processors if I don't provide the --parallel option or if I provide it with no argument. So the message is misleading.

Perhaps the error message should say:

You should re-run this test with the "--parallel 1" option to reproduce the failure and get a correct traceback.

I'll be happy to submit a PR if this ticket is accepted.

[Aymeric Augustin]

I believe this message can show up:

• when you run Django's test suite: runtests.py enables parallelization by default
• when you run a project's test suite: parallelization isn't enabled by default

The message is appropriate for the latter case but not for the former.

--parallel=1 covers both use cases. That's a good solution.

[Adam Zapletal]

PR is here: ​https://github.com/django/django/pull/7037

Thanks!

[Tim Graham <timograham@…>]

In 2a11d2d:

Fixed #27028 -- Clarified failed parallel tests message.

[Tim Graham <timograham@…>]

In 06ba4926:

[1.10.x] Fixed #27028 -- Clarified failed parallel tests message.

Backport of 2a11d2d7a7d5c6609c85dbc631fad6b8a8645a64 from master