Fix build with `-DBUILD_TESTING=OFF`
Fix build with
cmake -DBUILD_TESTING=OFF .. ... CMake Error at kdevplatform/cmake/modules/KDevPlatformMacros.cmake:124 (add_library): Target "kdevcustombuildsystem" links to target "KDev::Tests" but the target was not found. Perhaps a find_package() call is missing for an IMPORTED target, or an ALIAS target is missing? Call Stack (most recent call first): plugins/custom-buildsystem/CMakeLists.txt:21 (kdevplatform_add_plugin) -- Generating done CMake Generate step failed. Build files cannot be regenerated correctly.