Lowercase phpunit folder

This little detail is bugging me for some time. The folder is lowercased everywhere, even [in WikibaseQuery](https://github.com/wmde/WikibaseQuery/tree/master/tests).

Yes, the `Fixtures` and `Integration` folders next to the `Phpunit` folder are still uppercase. I think they should be lowercase too but I did not wanted to make this patch to complicated. Will do this in later patches if it's ok for you.