trial runs unit tests in an arbitrary order

[zooko]

It would sometimes be nice to run unit tests in a certain order, for example to test your hypothesis that one unit tests is screwing up the state of another. However, if you pass multiple tests on the command-line to trial, it will run them in an arbitrary order, perhaps by sticking them into a set and then iterating it, or the like.

[therve]

I think it's a duplicate of #1948.