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`.
The target property introduced by commit 413b71485a (Xcode: Create Xcode
schemes per target, 2019-03-11, v3.15.0-rc1~347^2) was accidentally not
initialized by `CMAKE_XCODE_GENERATE_SCHEME` for custom targets. Fix it
and update the test.
Fixes: #19759
Add `XCODE_SCHEME_*` target properties and associated variables
`CMAKE_XCODE_SCHEME_*` to initialize them on target creation.
Map each target property value to an associated Xcode scheme entry.
Co-Author: Martin Sander <mail@martin-sander.de>
Fixes: #17919