Try to improve tests/CMakeLists.txt usage

Currently developers have to duplicate find_package calls in both repo/CMakeLists.txt and in repo/tests/CMakeLists.txt to support standalone tests.

Investigate if it's possible to generate an extra Config.cmake file when configuring the top-level repo, which would contain the necessary find_package calls, and then that file gets reused inside the repo/tests/CMakeLists.txt standalone project.

This should allow us to remove duplication of find_package calls, and simplify the project structure.

Coin could then invoke CMake on the top level CMakeLists.txt project with a special -DQT_BUILD_STANDALONE_TESTS, which would make qt_build_repo and friends only add_subdirectory the tests, without building anything else.