Add CMake Integration tests

[SimeonEhrig]

I found played around with Unix IPC sockets this week and used the library therefore: https://github.com/fpagliughi/sockpp

Using the library via find_package works without problem. If I use the FetchContent from CMake, the CMake target does not provide the path of the include folder. I think the problem is caused by a missing INTERFACE target library. Long story short, if one method to integrate alpaka in the CMake project is working, it does not mean that the other methods also works. Therefore we should implement CMake integration tests, where an external Project uses alpaka via the following CMake methods:

• find_package()
• add_subdirectory()
• FetchContent_Declare() and FetchContent_MakeAvailable()

I did it already for vikunja: https://github.com/alpaka-group/vikunja/blob/9613b24ef5f9e0d66ef6d33e1781700c38d6f56c/ci/gitlab_scripts/integration_test/addsubdirectory.cmake#L47