Move include(CTest) to tests/CMakeLists.txt, or add it under include guard RTAUDIO_BUILD_TESTING

[Wallby]

Right now, include(CTest) in the top level CMakeLists.txt causes CTest targets to automatically generate even if having set set(RTAUDIO_BUILD_TESTING OFF).

[Raffaello]

this should be already resolved in master:
https://github.com/thestk/rtaudio/blob/master/CMakeLists.txt#L300-L303