Xcode: Restore CMAKE_XCODE_GENERATE_SCHEME for custom targets

Brad Kingrequested to merge
brad.king/cmake:xcode-scheme-custom-target into master

The target property introduced by !3098 (merged) was accidentally not initialized by CMAKE_XCODE_GENERATE_SCHEME for custom targets. Fix it and update the test.

Fixes: #19759 (closed)

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