Tests: Cover generate_apple_platform_selection_file on all platforms

The test added by commit 37bc3400cd (CMakePackageConfigHelpers: Add
generate_apple_platform_selection_file(), 2023-11-03) covers importing
only on macOS, and is specific to the xcframework test case.  Add a
dedicated test for `generate_apple_platform_selection_file` that covers
export and import on all platforms.
27 files changed