[Sofa.Testing] Add SimpleApi in Config.cmake.in

[alxbilger]

Following #4495
This is to fix https://github.com/sofa-framework/BeamAdapter/actions/runs/8069371516/job/22044285700?pr=140

CMake Error at BeamAdapter_test/CMakeLists.txt:5 (find_package):
  Found package configuration file:

    D:/a/BeamAdapter/BeamAdapter/sofa/lib/cmake/Sofa.Testing/Sofa.TestingConfig.cmake

  but it set Sofa.Testing_FOUND to FALSE so package "Sofa.Testing" is
  considered to be NOT FOUND.  Reason given by package:

  The following imported targets are referenced, but are missing:
  Sofa.SimpleApi



-- Configuring incomplete, errors occurred!

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