Steps to reproduce:
1. Create a new project
2. Open TestRunner
3. Hit "Create Test Assembly Folder" multiple times repeatedly (without going to Project view and changing the new item's name!)
4. Observe a hierarchy of folders and files all named "Tests", plus errors logged to console.

Expected result: When you're already inside a folder containing an Assembly Definition for unit tests, the button should be disabled

Reproduced in: 2018.2.0a1, 2018.1.0b5