Add CMake files for the unit tests and the testbed app

[pierre-dejoue]

In those commits building the unit tests and the testbed via cmake is optional, OFF by default, to prevent the clients of the library from pulling their dependencies (boost, glfw, etc.)

Running the unit tests was added to the Github Actions.

A note for Windows users: The tesbed has a dependency on GLFW, and CMake was not able find the package glfw3 from the pre-compiled binaries available for version 3.3.2, I ended up doing the following:

• Build GLFW from sources.
• Make the 'install' target as well (which will generate the proper cmake files, glfw3Config.cmake, glfw3Targets.cmake, etc.)
• Set the glfw3_ROOT env variable to point to the cmake files.
• find_package(glfw3 3.3 REQUIRED) should now work.

[jhasse]

Awesome stuff! Thank you :)

[pierre-dejoue]

You're welcome ;) Thanks for the merge!