Running test_standarddir would pollute the user's home with
`~/.cache/qute_test`.
The `no_cachedir_tag` fixture was supposed to prevent this, but was not
working because [usefixtures does not work on fixtures]
(https://github.com/pytest-dev/pytest/issues/1014).
This fixes the fixture to actually prevent cachedir creation, but
applies it to tests individually (or by class) rather than with autouse
because the cachedir tests cannot pass if it is working.
1f71520bb2