Fixed GTest find command and re-enabled tests.

[LinusVanElswijk]

CMake could not find GTest, because of a small typo in the CMakeFile ("Gtest" instead of "GTest"). This commit fixes that and adds the 'tests' subdirectory back into to the project. (It was commented out before).

If GTest cannot be found, the test subdirectory is automatically skipped.

[andre-schulz]

Hi @LinusVanElswijk, thanks for your contribution. I will take a closer look at it as soon as I can.

[andre-schulz]

Hi @LinusVanElswijk, sorry for the long delay. :( Overall, this looks pretty good and I would request only a few minor changes ... that is if you're still interested in this.