Move Test cases under project root dir 

This PR aims to reduce the size of the releases uploaded to pypi, and also acknowledge that running tests isn't possible with default Scrapy requirements, so it doesn't make sense to ship them to final installations anymore.

After this changes the source distribution is 85% its previous size (636KB vs 750KB), and the Wheel distribution is 57% its previous size (280KB vs 491KB)


I think it won't work when "scrapy bench" is executed by users.