Tests/RunCMake: Update cmake_minimum_required versions

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`.

Tests/RunCMake/XcodeProject/XcodeObjcFlags.cmake

@@ -1,5 +1,5 @@
-cmake_minimum_required(VERSION 3.15)
-project(objctest LANGUAGES C OBJC)
+enable_language(C)
+enable_language(OBJC)
 
 include(CheckOBJCCompilerFlag)
 check_objc_compiler_flag(-fobjc-arc HAVE_OBJC_ARC)