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`.
Add source file properties to control Xcode file type attributes:
XCODE_EXPLICIT_FILE_TYPE => explicitFileType
XCODE_LAST_KNOWN_FILE_TYPE => lastKnownFileType
Add a RunCMake.XcodeProject test to verify generated project content.
