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`.

`CMAKE_<LANG>_COMPILER` may be a list if it was defined by a toolchain
file.  In this case we move the args to `CMAKE_<LANG>_COMPILER_ARG1`.
If `CMAKE_<LANG>_COMPILER` is not a list, then it might have been cached
by a previous run that split the `CC` or `CXX` environment variable into
`CMAKE_<LANG>_COMPILER` and `CMAKE_<LANG>_COMPILER_ARG1`.  In this
latter case, avoid clobbering `CMAKE_<LANG>_COMPILER_ARG1`.

Fixes: #23358

Also distinguish between the toolchain and env cases in the output.