Shared library stubs can be used for linking, but not at runtime.
Their role is similar to import libraries on Windows, so represent
their location with the `IMPORTED_IMPLIB` target property.
Fixes: #24940
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`.
Brad King