Well to test tag_sorter, what do you need? The test should contain as little code as possible that is not related to its primary goal: test a certain function.

In this case, you have a lot of code which in test_tag_sorter is in fact testing another function (add_tag), while you have at the top level of the tests a number of methods which might provide you the environment you need to perform your testing.

In this case, to test tag_sorter you need tags, is the function create_tag enough to test tag_sorter? If so then use it and focus test_tag_sorter around tag_sorter, not around add_tag and tag_sorter.