[MacOS] Generating XCode solution fails

[LB-MateuszSzweda]

Describe the bug
It has been observed that generating XCode solution for the build will fail shortly after successful configuration with
the following error:

CMake Error in AutomatedTesting/Gem/Code/CMakeLists.txt:
Target "AutomatedTesting" has source files which vary by configuration.
This is not supported by the "Xcode" generator.

If the AutomatedTesting project is not specified as a project in configuration step the generated error is:

CMake Error in Code/Framework/AtomCore/CMakeLists.txt:
Target "AtomCore.Tests" has source files which vary by configuration. This
is not supported by the "Xcode" generator.

This issue occurs when configuring and generating XCode solutions with both
cmake .. -G "Xcode" -DLY_3RDPARTY_PATH=$LY_3RDPARTY_PATH -DLY_PROJECTS=AutomatedTesting -DLY_UNITY_BUILD=1
command and CMake GUI preventing the user from building projects.

For further information please refer to the attached configure log files.
XCode Generation failed.txt
XCode Generation without specified project.txt

To Reproduce
Steps to reproduce the behavior:

1. Use Github to obtain the latest O3DE build on your Mac.
2. Create a new or move your preexisting 3rdParty folder into the O3DE folder.
3. Obtain Cmake.app and move it to the created 3rdParty/CMake directory (create
   the CMake folder if necessary).
4. In the O3DE directory right-click on python folder and select New Terminal at Folder.
5. Run the command:
   export PATH=$PATH:/Users//o3de/3rdParty/CMake/CMake.app/Contents/bin
6. Run the command:
   ./get_python.sh
7. Create a folder named osx_build in the O3DE folder.
8. Open a new terminal in the created osx_build folder and set the following temporary environment variables:

   export PATH=$PATH:/Users//o3de/3rdParty/CMake/CMake.app/Contents/bin
   export LY_3RDPARTY_PATH=/Users//o3de/3rdParty
   export LY_PACKAGE_SERVER_URLS=https://d2c171ws20a1rv.cloudfront.net
   

9. Configure and generate Xcode solution:
   cmake .. -G "Xcode" -DLY_3RDPARTY_PATH=$LY_3RDPARTY_PATH -DLY_PROJECTS=AutomatedTesting -DLY_UNITY_BUILD=1

Expected behavior
XCode solution is successfully generated.

Desktop/Device:
- Device: Mac
- OS: MacOS
- Version: Big Sur 11.3.1
- CPU: Intel Core i7 2,8 GHz
- GPU: Radeon Pro 555
- Memory: 16GB

[LB-MateuszSzweda]

The issue was verified as fixed on both Development and Stabilization/2110 branch and no longer occurs for over 2 months.