collectionattributetest: small cleanups

Summary:
* Don't fail with "duplicate collection" errors after a failure in one
of the tests, properly cleanup
* Avoid "no such collection" warning due to knut modifying the
collection after we deleted it
* Fix typo in method name

Test Plan: test still passes

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D19292