We use built-in unittest (or Django's wrappers) for testing a Python project. 
In some of those tests, we use libs like freezegun or mock, which aren't used anywhere in the production codebase. 
Our CI that runs tests installs all deps before a test run, so usually we'd put those in dev-deps. 

Is it common to leave those in the dev-packages section of the Pipfile, or should test-related packages also reside in packages?
a small note about mock since Python v3.3 it's part of the unittest module.
Said that, in theory would be better to keep those kind of packages in the dev-dependencies.
In practice you could ignore the problem unless you

you have tons of dependencies
some dependency is hard to install (maybe it requires a C compiler installed or something similar)
If I only use a library during test suite runs, should it be in normal or dev packages in my Pipfile?