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