Permit to test when building bindings separately from main library

[traversaro]

🦟 Bug fix

Follow up of #1491 . #1491 enable the compilation of bindings separately from the main library, but does not permit to set BUILD_TESTING:BOOL=ON in those binding-only builds.

Summary

This PR permits to pass -DBUILD_TESTING:BOOL=ON in bindings-only builds, by:

• Ensuring that ${PROJECT_BINARY_DIR}/include/test_config.hh is generated also in bindings-only builds,
• Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR when creating the tests, as the CMAKE_CURRENT_SOURCE_DIR variable have a consistent value that is not influenced on which is the root directory of the CMake build.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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