For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
Create a <LANG>_INCLUDE_WHAT_YOU_USE target property (initialized by a
CMAKE_<LANG>_INCLUDE_WHAT_YOU_USE variable) to specify an IWYU command
line to be run along with the compiler.