Fix test_unique_slug: speed it up
Fix test_unique_slug: speed it up

The fix: instead of double calling the client, we can test for unique
slug by adding the Event with repeated slug.

vahtras/amy,swcarpentry/amy,vahtras/amy,swcarpentry/amy,pbanaszkiewicz/amy,swcarpentry/amy,vahtras/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy