switch to cmake gtest capabilities

[cgcostume]

CMake now seems to support google test out of the box (since 3.10 or 3.9. something). This might reduce the need to include google test and mock within the glbinding repository. Furthermore, all major IDEs and systems provide google test by default (pre-installed, even in Visual Studio since 2017 or even earlier).

[cgcostume]

https://cmake.org/cmake/help/v3.14/module/GoogleTest.html
https://blog.kitware.com/dynamic-google-test-discovery-in-cmake-3-10/

[scheibel]

Before this, we need to decide on the minimum required CMake version for cmake-init. This is currently CMake 3.2 with some backward-compatibility layers for CMake down to 3.0.