Unit tests for printhooks
CreateTestsEvent used to change Event._attrs in class initialization section (executed on module import) which lead to CreateTestsEvent's specific attributes appearing in _attrs of all the other events. This effect was apparent in printhooks' unit tests.

I believe it was a simple typo, thus changed.
As per the docs ("Using nose2"), `--print-hooks` switch should also have short form `-P`
Basic functional tests for printhooks plugin
Merge pull request #185 from kurniliya/printhooks-tests

Printhooks tests