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`.
This allows for setting EXCLUDE_FROM_ALL, conditional on the build
configuration. However, only the Ninja Multi-Config generator supports
different property values per config. All other multi-config
generators will yield an error in that situation.
Fixes: #20923