I'm working on this in the PyCon 2019 sprints.

Near as I can tell, while this issue still seems relevant, I think it might actually be for the best that this multiple disambiguation is left in the test suite. 

I removed the pid reference in the TESTFN and the tests passed in both the parallel and sequential cases. However, removing the pid results in multiprocessing tests having to be written more carefully if they choose to use the TESTFN.

Here is an explanation for why you would leave this code in. When running the tests in sequential mode, the tests will run in a CWD that includes the pid. When writing a multi-processing test using the TESTFN you would have to remember to add the os.getpid call to the actual TESTFN instead of it just being included by default.

Ultimately, this is really just extra information that is included in the temporary filenames. It might be worth just closing this issue.